s390/vdso: Get rid of -m64 flag handling

s390/vdso: Get rid of -m64 flag handling

The compiler/assembler flag -m64 is added and removed at two locations.
This pointless exercise is a leftover to keep the 31 and 64 bit vdso
Makefiles as symmetrical as possible. Given that the 31 bit vdso code
does not exist anymore, remove the -m64 flag handling.

Suggested-by: Jens Remus <jremus@linux.ibm.com>
Reviewed-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>