x86emul/test: wrap libc functions with FPU save/restore code

x86emul/test: wrap libc functions with FPU save/restore code

Currently with the native tool chain on Debian Jessie ./test_x86_emulator
yields:

  Testing AVX2 256bit single native execution...okay
  Testing AVX2 256bit single 64-bit code sequence...[line 933] failed!

The bug is that libc's memcpy() in read() uses %xmm8 (specifically, in
__memcpy_sse2_unaligned()), which corrupts %ymm8 behind the back of the AVX2
test code.

Introduce wrappers (and machinery to forward calls to those wrappers)
saving/restoring FPU state around certain library calls.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-and-tested-by: Andrew Cooper <andrew.cooper3@citrix.com>