s390/fpu: Use CONFIG_CC_HAS_ASM_AOR_FORMAT_FLAGS instead of CONFIG_CC_IS_CLANG

Use the more precise CONFIG_CC_HAS_ASM_AOR_FORMAT_FLAGS to tell if the
compiler has support for the A, O, and R inline assembly format flags.
This allows recent Clang compilers to generate better code.

Move code around so the good (aka better) case at the top of each ifdef
construct.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>