Validate failures (perf, ghc-7.8)

[Edward Z. Yang]

I've got perf failures on Linux 64-bit on ghc-7.8 (and master too, I
think).  Please take a look.

Unexpected results from:
TEST="T4801 T3064 T5837 T6048"
  
OVERALL SUMMARY for test run started at Thu Apr 10 15:33:27 2014 PDT
 0:02:52 spent to go through
    3920 total tests, which gave rise to
   12853 test cases, of which
    9274 were skipped
  
      26 had missing libraries
    3491 expected passes
      58 expected failures
  
       0 caused framework failures
       0 unexpected passes
       4 unexpected failures
  
Unexpected failures:
   perf/compiler  T3064 [stat not good enough] (normal)
   perf/compiler  T4801 [stat not good enough] (normal)
   perf/compiler  T5837 [stat not good enough] (normal)
   perf/compiler  T6048 [stat not good enough] (optasm)