Newer nightlies check a new environment variable that if set will loosen
restrictions on which compiler version can be used for bootstrapping.
Upstream issue is at https://github.com/rust-lang/rust/pull/37265
The OS is identified as "10.4" rather than "osx". This commit removes
the 'rt' build script's hard coding of the "osx" suffix when building on
darwin since the cmake configuration falls back to building a 10.4
compatible libclang_rt.