src/bootstrap/cc.rs: Support specifying AR in config or via environment variable

[devurandom]

src/bootstrap/cc.rs currently derives AR from CC via build_helper.cc2ar(). However, it would be very helpful if AR could be specified in config.toml, like CC can. Alternatively, it would be desirable to be able to provide the binary name via a environment variable.

[alexcrichton]

Makes sense to me! Should be relatively easy to implement, and if anyone's interested in doing so I don't mind helping out!

[petrochenkov]

I have a branch on which archiver and linker can be supplied to rustbuild explicitly, will send a PR soon.