s390: make use of CONFIG_FUNCTION_ALIGNMENT

Make use of CONFIG_FUNCTION_ALIGNMENT which was introduced with commit
d49a0626 ("arch: Introduce CONFIG_FUNCTION_ALIGNMENT").

Select FUNCTION_ALIGNMENT_8B for gcc in order to reflect gcc's default
function alignment. For all other compilers, which is only clang, select
a function alignment of 16 bytes which reflects the default function
alignment for clang.

Also change the __ALIGN define to follow whatever the value of
CONFIG_FUNCTION_ALIGNMENT is. This makes sure that the alignment of C and
assembler functions is the same.

In result everything still uses the default function alignment for both
compilers. However in addition this is now also true for all assembly
functions, so that all functions have a consistent alignment.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>