HEAD bootstrap error macOS

[Gabor Greif]

Recently I started getting following error:
```
[nix-shell:~/ghc]$ "inplace/bin/ghc-stage1" -optc-Wall -optc-Wall
-optc-Wextra -optc-Wstrict-prototypes -optc-Wmissing-prototypes
-optc-Wmissing-declarations -optc-Winline -optc-Wpointer-arith
-optc-Wmissing-noreturn -optc-Wnested-externs -optc-Wredundant-decls
-optc-Wno-aggregate-return -optc-Iincludes -optc-Iincludes/dist
-optc-Iincludes/dist-derivedconstants/header
-optc-Iincludes/dist-ghcconstants/header
-optc-Iincludes/dist-install/build -optc-Irts -optc-Irts/dist/build
-optc-DCOMPILING_RTS -optc-DFS_NAMESPACE=rts -optc-fno-strict-aliasing
-optc-fno-common -optc-DDTRACE -optc-Irts/dist/build/./autogen
-optc-Wno-unknown-pragmas -optc-O2 -optc-fomit-frame-pointer -optc-g
-optc-DRtsWay=\"rts_thr\" -static -optc-DTHREADED_RTS  -O0 -H64m -Wall
  -Iincludes -Iincludes/dist -Iincludes/dist-derivedconstants/header
-Iincludes/dist-ghcconstants/header -Iincludes/dist-install/build
-Irts -Irts/dist/build -DCOMPILING_RTS -DFS_NAMESPACE=rts
-this-unit-id rts -dcmm-lint  -DDTRACE     -i -irts -irts/dist/build
-Irts/dist/build -irts/dist/build/./autogen -Irts/dist/build/./autogen
           -O2 -Wcpp-undef    -Wnoncanonical-monad-instances  -c
rts/sm/Scav_thr.c -o rts/dist/build/sm/Scav_thr.thr_o -v
Glasgow Haskell Compiler, Version 8.11.0.20191222, stage 1 booted by
GHC version 8.6.5
*** initializing package database:
Using binary package database:
/Users/ggreif/ghc/inplace/lib/package.conf.d/package.cache
package flags []
loading package database /Users/ggreif/ghc/inplace/lib/package.conf.d
wired-in package ghc-prim mapped to ghc-prim-0.6.1
wired-in package integer-wired-in mapped to integer-gmp-1.0.2.0
wired-in package base mapped to base-4.14.0.0
wired-in package rts mapped to rts
wired-in package template-haskell mapped to template-haskell-2.16.0.0
wired-in package ghc mapped to ghc-8.11.0.20191222
!!! initializing package database: finished in 8.51 milliseconds,
allocated 8.770 megabytes
Created temporary directory:
/var/folders/39/lr50t0q96tx5qp0mzht34r240000gn/T/ghc55616_0
*** systool:cc:
*** C Compiler:
/nix/store/mq8dqgqf2qqkp77pzf3jwc05px8rkbny-clang-wrapper-9.0.0/bin/cc
-x c rts/sm/Scav_thr.c -o
/var/folders/39/lr50t0q96tx5qp0mzht34r240000gn/T/ghc55616_0/ghc_1.s
-fno-common -U__PIC__ -D__PIC__ -Wimplicit -S -O2 -include
/Users/ggreif/ghc/includes/ghcversion.h -Iincludes -Iincludes/dist
-Iincludes/dist-derivedconstants/header
-Iincludes/dist-ghcconstants/header -Iincludes/dist-install/build
-Irts -Irts/dist/build -Irts/dist/build -Irts/dist/build/./autogen
-I/Users/ggreif/ghc/libraries/base/include
-I/Users/ggreif/ghc/libraries/base/dist-install/build/include
-I/Users/ggreif/ghc/libraries/base/dist-install/build/dist-install/build/include
-I/Users/ggreif/ghc/libraries/integer-gmp/include
-I/Users/ggreif/ghc/libraries/integer-gmp/dist-install/build/include
-I/Users/ggreif/ghc/libraries/integer-gmp/dist-install/build/dist-install/build/include
-I/Users/ggreif/ghc/rts/dist/build -I/Users/ggreif/ghc/includes
-I/Users/ggreif/ghc/includes/dist-derivedconstants/header
-I/Users/ggreif/ghc/includes/dist-install/build -Xpreprocessor
-DCOMPILING_RTS -Xpreprocessor '-DFS_NAMESPACE=rts' -Xpreprocessor
-DDTRACE -Wall -Wall -Wextra -Wstrict-prototypes -Wmissing-prototypes
-Wmissing-declarations -Winline -Wpointer-arith -Wmissing-noreturn
-Wnested-externs -Wredundant-decls -Wno-aggregate-return -Iincludes
-Iincludes/dist -Iincludes/dist-derivedconstants/header
-Iincludes/dist-ghcconstants/header -Iincludes/dist-install/build
-Irts -Irts/dist/build -DCOMPILING_RTS '-DFS_NAMESPACE=rts'
-fno-strict-aliasing -fno-common -DDTRACE -Irts/dist/build/./autogen
-Wno-unknown-pragmas -O2 -fomit-frame-pointer -g '-DRtsWay="rts_thr"'
-DTHREADED_RTS

In file included from rts/sm/Scav_thr.c:3:0: error:

In file included from rts/sm/Scav.c:51:0: error:

In file included from rts/sm/GCUtils.h:18:0: error:

rts/sm/GCTDecl.h:113:1: error:
     error: register '%r13' unsuitable for global register variables
on this target
    |
113 | GCT_REG_DECL(gc_thread*, gct, REG_Base);
    | ^
GCT_REG_DECL(gc_thread*, gct, REG_Base);
^

rts/sm/GCTDecl.h:37:56: error:
     note: expanded from macro 'GCT_REG_DECL'
   |
37 | #define GCT_REG_DECL(type,name,reg) register type name REG(reg);
   |                                                        ^
#define GCT_REG_DECL(type,name,reg) register type name REG(reg);
                                                       ^

includes/stg/MachRegs.h:160:24: error:
     note: expanded from macro 'REG'
    |
160 | #define REG(x) __asm__("%" #x)
    |                        ^
#define REG(x) __asm__("%" #x)
                       ^
1 error generated.
*** Deleting temp files:
Deleting: /var/folders/39/lr50t0q96tx5qp0mzht34r240000gn/T/ghc55616_0/ghc_1.s
/var/folders/39/lr50t0q96tx5qp0mzht34r240000gn/T/ghc55616_0/ghc_2.rsp
Warning: deleting non-existent
/var/folders/39/lr50t0q96tx5qp0mzht34r240000gn/T/ghc55616_0/ghc_1.s
*** Deleting temp dirs:
Deleting: /var/folders/39/lr50t0q96tx5qp0mzht34r240000gn/T/ghc55616_0
`cc' failed in phase `C Compiler'. (Exit code: 1)
```
My $CC is clang-9 (but I also tried others).

Has anybody seen something similar?

Cheers nd thanks,

     Gabor