with respect to std.builtin.link_libc.

The commit 27e008eb292038c5a6b9a13b64c7b69d1525f690 did not solve the
problem because although it got std.builtin.link_libc to be true for
compiler_rt.zig and c.zig, it had other unintentional side effects which
broke the build for -lc -target foo-linux-musl.

This commit introduces a new flag to Compilation to allow setting this
comptime flag to true without introducing other side effects to
compilation and linking.