clang: Don't look for libgcc

clang: Don't look for libgcc

In the case of using clang to build, and having not already enabled the
private libgcc, do not look for it, as it will not be found nor
required.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>