Add support for -fno-plt

[GabrielMajeri]

Similar to rust-lang/rust#54592, this PR enables the -fno-plt compiler flag by default when PIC is used. Should provide a small performance improvement with no obvious downsides.

[GabrielMajeri]

Uh, I've run into a problem: how do I check if flag_is_supported from inside try_get_compiler? Because it seems it causes an infinite loop.

[alexcrichton]

Hm so unfortunately is_flag_supported is somewhat expensive, could this perhaps default to off for now?

[GabrielMajeri]

@alexcrichton Ok, I've made it so that this flag is not enabled by default. Another alterantive would've been to look at GCC's / Clang's version directly, and hard-code a version check, but it looks like we don't retrieve the compiler's version at any point.

[alexcrichton]

Ok thanks!