testsuite: Skip c-c++-common/zero-scratch-regs-10.c on arm

As discussed in PR 97680, -fzero-call-used-regs is not supported on
arm.

Skip this test to avoid failure reports.

2021-03-18  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/testsuite/
	PR testsuite/97680
	* c-c++-common/zero-scratch-regs-10.c: Skip on arm