Disable x87 inline functions for SSE2 math

When i386 and x86-64 mathinline.h was merged into a single mathinline.h,
"gcc -m32" enables x87 inline functions on x86-64 even when -mfpmath=sse
and SSE2 is enabled.  It is a regression on x86-64.  We should check
__SSE2_MATH__ instead of __x86_64__ when disabling x87 inline functions.