s390/linkage: increase asm symbols alignment to 16

Both clang and gcc (for -march=z13 and later) align functions to 16
bytes at -O2 to benefit branch prediction.

Make asm symbols alignment consistent with that.

This also benefits potential ftrace code patching, which is currently
able to patch 8 aligned bytes at once.

With defconfig this currently increases .text size by 4104 bytes.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>