xtensa: don't link with libgcc

libgcc may be absent or may have different ABI than the kernel. Don't
link with it. Drop declarations and export for helpers that are not
implemented. Use generic versions of DI mode multiplication and
comparisons. Drop register window spilling helper as it should never be
used by the compiler-generated code.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>