The CFLAGS_REMOVE_<file> feature allows default C compiler flags to be
removed for particular files. Add the same feature for assembler, using
AFLAGS_REMOVE_<file>.

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