i386 -fPIC requires `%ebx`

i386 -fPIC requires `%ebx`

Summary:On i368 (until, I'm lead to believe i686) `%ebx` is used for the PIC register. Thus, when we use cpuid, we clobber that register and gcc loudly complains:

  folly/CpuId.h: In member function 'virtual void CpuId_Simple_Test::TestBody()':
  folly/CpuId.h:67:61: error: PIC register clobbered by 'ebx' in 'asm'
       __asm__("cpuid" : "=a"(n) : "a"(0) : "ebx", "edx", "ecx");

Reviewed By: yfeldblum

Differential Revision: D2991229

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