Choose target features

[sanxiyn]

Fix#1879.

[sanxiyn]

Given:

// test.rsfntest(a:float,b:float) -> float{ a * b }fnmain(){}

# On 32-bit x86, SSE2 is preferred if available
$ rustc -S test.rs -o - | grep mul
fmulp
$ rustc --target-feature +sse2 -S test.rs -o - | grep mul
mulsd