ARM: Fix Makefile during SPL and TPL build

The tiny variants of memset and memcpy implementations can be
built for TPL as well, check whether a TPL build is in progress
and avoid including the default variants.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>