s390/bug: make use of asm_inline

This is the s390 version of commit 32ee8230 ("x86: bug.h: use
asm_inline in _BUG_FLAGS definitions").

See commit eb111869 ("compiler-types.h: add asm_inline
definition") for more details.

Just like on x86 the .text section size decreases a bit while the
.data section size increases about the same amount (gcc 9.2 with
defconfig).

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>