arm: Use the WEAK assembly entry point consistently

It is a bad idea, and more modern toolchains will fail, if you declare
an assembly function to be global and then weak, instead of declaring it
weak to start with. Update assorted assembly files to use the WEAK macro
directly.

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