IBM zSystems: Fix function_ok_for_sibcall [PR106355]

IBM zSystems: Fix function_ok_for_sibcall [PR106355]

For a parameter with BLKmode we cannot use REG_NREGS in order to
determine the number of consecutive registers.  Streamlined this with
the implementation of s390_function_arg.

Fix some indentation whitespace, too.

gcc/ChangeLog:

	PR target/106355
	* config/s390/s390.cc (s390_call_saved_register_used): For a
	parameter with BLKmode fix determining number of consecutive
	registers.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/pr106355.h: Common code for new tests.
	* gcc.target/s390/pr106355-1.c: New test.
	* gcc.target/s390/pr106355-2.c: New test.
	* gcc.target/s390/pr106355-3.c: New test.

(cherry picked from commit cb994acc08b67f26a54e7c5dc1f4995a2ce24d98)