parallel build fixes for 7.4.2

[Conrad Parker]

Hi,

recently we've been tweaking our internal build system at Tsuru to
handle parallel builds of both cabal packages via 'cabal-sort
--makefile' and our local code tree via 'ghc -M'. In addition to the
recompilation checker fixes of #5878, the following would be great to
have in 7.4.2:

1) http://hackage.haskell.org/trac/ghc/ticket/5891 -- the patch fixes
a race condition in creating parent directories for built object files

2) master commit b6f94b5 "Compile link .note section separately from
main.c" -- I think this is the patch that fixes link errors we've seen
during parallel builds (via ghc -M) with 7.4.1, such as:

/x/y/z.o: file not recognized: File truncated

and:

/usr/bin/ld: BFD (GNU Binutils for Ubuntu) 2.20.51-system.20100908
internal error, aborting at ../../bfd/merge.c line 872 in
_bfd_merged_section_offset

Will everything currently in master already be included in the next
release or is it a separate branch? (If it's a separate branch I'll do
some more testing to confirm that b6f94b5 is the patch that fixes the
link error).

Conrad.