compiler-builtins: eliminate a directory symlink

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]`.

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

Author: tgross35

library/compiler-builtins/compiler-builtins/src/math/libm_math

@@ -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 {