Fix test-fenv:test_dfl_env when run on some amd64 CPUs

Fix test-fenv:test_dfl_env when run on some amd64 CPUs

Compare the fields that the AMD [1] and Intel [2] specs say will be
set once fnstenv returns.

Not all amd64 capable processors zero out the env.__x87.__other field
(example: AMD Opteron 6308). The AMD64/x64 specs aren't explicit on what the
env.__x87.__other field will contain after fnstenv is executed, so the values
in env.__x87.__other could be filled with arbitrary data depending on how the
CPU-specific implementation of fnstenv.

1. http://support.amd.com/TechDocs/26569_APM_v5.pdf
2. http://www.intel.com/Assets/en_US/PDF/manual/253666.pdf

Discussed with: kib, Anton Rang <anton.rang@isilon.com>
Reviewed by: Daniel O'Connor <darius@dons.net.au> (earlier patch; pre-generalization)
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division
Reported by: Bill Morchin <wmorchin@isilon.com>