Skip building libclang_rt when WITHOUT_CLANG is used

As noted in bug 277096, when building a pkgbase repository using
WITHOUT_CROSS_COMPILER and WITHOUT_TOOLCHAIN (which sets WITHOUT_CLANG),
the following residual files are left over:

/usr/lib/clang/18/lib/freebsd/libclang_rt.asan-x86_64.so
/usr/lib/clang/18/share/asan_ignore_list.txt
/usr/lib/clang/18/share/cfi_ignore_list.txt
/usr/lib/clang/18/share/msan_ignore_list.txt

This is because the lib/libclang_rt directory is still descended into,
even if WITHOUT_CLANG is used. Fix it by not descending into the
libclang_rt directory in that case.

PR: 277096
Reported by: Siva Mahadevan <me@svmhdvn.name>
MFC after: 3 days