s390: Add CC_HAS_ASM_IMMEDIATE_STRINGS

Upcoming changes to s390 specific inline assemblies require the usage of
strings as immediate input operands. This works only with gcc-9 and newer
compilers. With gcc-8 this leads to a compile error:

void bar(void) { asm volatile("" :: "i" ("foo")); }

Results in:

In function 'bar':
 warning: asm operand 0 probably doesn't match constraints
  asm volatile("" :: "i" ("foo"));
  ^~~
 error: impossible constraint in 'asm'

Provide a CC_HAS_ASM_IMMEDIATE_STRINGS config option which allows to tell
if the compiler supports strings as immediate input operands.
Based on that conditional code can be provided.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>