testsuite: Fix up pr98920.c on non-glibc or old glibc targets [PR98920]

testsuite: Fix up pr98920.c on non-glibc or old glibc targets [PR98920]

Not all OSes have regex.h and not all OSes that do have REG_STARTEND macro support.
Conditionalize the test on that.

2021-03-09  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/98920
	* c-c++-common/asan/pr98920.c: Only include regex.h if the header
	exists.  If REG_STARTEND macro isn't defined, just return 0 from main
	instead of the actual test.