It is already possible to remove CFLAGS with the CFLAGS_REMOVE option
that was introduced with commit 656ee82c ("kbuild: create new
CFLAGS_REMOVE_(basename).o option").  However it is not possible to
remove AFLAGS for assembler files.

So this patch just adds the AFLAGS_REMOVE option which works the same
like CFLAGS_REMOVE.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Michal Marek <mmarek@suse.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>