[lldb] [Process/NetBSD] Remove unnecessary FPU presence checks for x86_64

Remove the checks for FPU presence, FXSAVE support and usage from
the code for x86_64.  Those are always true for this architecture,
and in fact are hardcoded to true inside NetBSD kernel.

Differential Revision: https://reviews.llvm.org/D63554

llvm-svn: 363823