testsuite: Fix up gcc.target/s390/zero-scratch-regs-1.c

Depending on whether GCC is configured using --with-mode=zarch or not,
for the 31bit target instructions are generated either for ESA or
z/Architecture.  For the sake of simplicity and robustness test only for
the latter by adding manually option -mzarch.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/zero-scratch-regs-1.c: Force test to run for
	z/Architecture only.