Choose target features

[sanxiyn]

Fix #1879.

[sanxiyn]

Given:

// test.rs
fn test(a: float, b: float) -> float { a * b }
fn main() {}

# 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