Unbreak folly on x86-32
Unbreak folly on x86-32

Summary:
_mm_cvtsi64_si128 and _mm_cvtsi128_si64 intrinsics are available on
x86_64 only, so do not enable SSE instructions when compiling for 32 bits.

Reviewed By: yfeldblum

Differential Revision: D6422271

fbshipit-source-id: a3c5cf3f2ee4b47d5a1edcf93cfff3d1f3fe907e

rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly,rklabs/folly