Support setting target-features and target-cpu

[gnzlbg]

It would be great to have a way to be able to forward enabled Rust target features down to the C and C++ compilers, mapping the Rust names to the C / C++ compiler names depending on the compiler being used (for clang and gcc these names should be identical to the Rust ones most of the time).

The same applies to the target CPU where different compilers use -mcpu/-march/... to set it.

[alexcrichton]

Seems reasonable to me!

[alexcrichton]

I'm actually going to close this in favor of #268