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>