s390: Implement clz and ctz for SI mode

s390: Implement clz and ctz for SI mode

To properly implement __builtin_ffs for SI mode, implement clz and
(for >= z17) ctz for SI mode.  Otherwise, gcc falls back to a libcall
which causes problems for Linux kernel code.

Also adjust the C?Z_DEFINED_VALUE_AT_ZERO macros to return 2.  Since
the optabs now return exactly the value set by these macros, return
value 2 is more appropriate and leads to better code.

gcc/ChangeLog:

	* config/s390/s390.h (CLZ_DEFINED_VALUE_AT_ZERO): Adjust and
	return 2.
	(CTZ_DEFINED_VALUE_AT_ZERO): Return 2.
	* config/s390/s390.md (clzsi2): Implement.
	(ctzsi2): Implement.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/pr109011-2.c: Fix expected outcome.
	* gcc.dg/vect/pr109011-4.c: Fix expected outcome.
	* gcc.target/s390/ffs-1.c: New test.

Signed-off-by: Juergen Christ <jchrist@linux.ibm.com>