testsuite: Require openmp effective-target for PR100515

The related test uses -fopenmp, which is not supported by newlib-based
targets such as arm-eabi or aarch64-elf.

Requiring the openmp effective-target makes the test unsupported
rather than failed.

2021-05-17  Christophe Lyon  <christophe.lyon@linaro.org>

	PR debug/100515
	gcc/testsuite
	* gcc.dg/debug/dwarf2/pr100515.c: Require openmp effective-target.