Currently we set dynamic-linker unconditionally. This breaks
however some static binaries i.e. rust binaries linked against musl.
There is no reason we should set an elf interpreter for static binaries
hence this is skipped if `-static` or `-static-pie` is either passed to
our cc or ld wrapper.
fc9644d4c9 accidentally enabled the
sanitizer tests for GCC on Darwin, when fixing that case was never
attempted. Also inverted the condition from broken to working for
clarity.
We already did them on non-mass-rebuild llvm 6. Also, this allows
simplifying the stdenv booting.
We were missing the libcxxabi dep in compile-rt in llvm 6, so fixed that
too.
