s390: pass march flag to assembly files as well

Currently the march flag gets only passed to C files, but not to
assembler files.
This means that we can't add new instructions like e.g. aghik to asm
files, since the assembler doesn't know of the new instructions if
the appropriate march flag isn't specified.

So also pass the march flag when compiling assembler files as well.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>