Just minor QA fixes suggested by pkgcheck.

This follows the package moves in ::gentoo after the introduction of the
llvm-core and llvm-runtimes categories.

move sys-libs/libcxx -> llvm-runtimes/libcxx
move sys-libs/libcxxabi -> llvm-runtimes/libcxxabi
move sys-devel/lld -> llvm-core/lld
move syys-devel/clang -> llvm-core/clang

This should speed up pkgcheck operation by storing the caches which
pkgcheck is currently re-generating on every job run.

With this we can get easier to read output when there are pkgcheck
issues.

Issue: gentoo/fem-overlay#317
Issue: gentoo/fem-overlay#316

It seems the current versions of this package cause compile failures
with recent Go versions.

Closes: #314

This patch was previously backported from a pull request which has now
been merged. Thus, the patch is no longer necessary.

Closes: gentoo/fem-overlay#252

Closes: gentoo/fem-overlay#249

Closes: gentoo/fem-overlay#250