compiler-builtins: Eliminate symlinks

compiler-builtins has a symlink to the `libm` source directory so the
two crates can share files but still act as two separate crates. This
causes problems with some sysroot-related tooling, however, since
directory symlinks seem to not be supported.

The reason this was a symlink in the first place is that there isn't an
easy for Cargo to publish two crates that share source (building works
fine but publishing rejects `include`d files from parent directories, as
well as nested package roots). However, after the switch to a subtree,
we no longer need to publish compiler-builtins; this means that we can
eliminate the link and just use `#[path]`.

Similarly, the LICENSE file was symlinked so it could live in the
repository root but be included in the package. This is also removed as
it caused problems with the dist job (error from bootstrap's
`tarball.rs`, "generated a symlink in a tarball").

If we need to publish compiler-builtins again for any reason, it would
be easy to revert these changes in a preprocess step.

Author: tgross35

library/compiler-builtins/compiler-builtins/LICENSE.txt

@@ -2,6 +2,7 @@
 #[allow(dead_code)]
 #[allow(unused_imports)]
 #[allow(clippy::all)]
+#[path = "../../../libm/src/math/mod.rs"]
 pub(crate) mod libm_math;
 
 macro_rules! libm_intrinsics {