multimedia/x264: don't require SSE on i386 with ASM=on (default)

Remove "-march=i686 -mfpmath=sse -msse" and "-msse2" on later versions
when building C files. This doesn't affect hand-written assembly which
relies on CPUID but may pessimize compiler (around 2.3 % on Skylake).

Differential Revision: https://reviews.freebsd.org/D7958
Approved by: koobs (maintainer)