arm/testsuite: Fix testcase for PR99977

arm/testsuite: Fix testcase for PR99977

Some targets (eg arm-none-uclinuxfdpiceabi) do not support Thumb-1,
and since the testcase forces -march=armv8-m.base, we need to check
whether this option is actually supported.

Using dg-add-options arm_arch_v8m_base ensure that we pass -mthumb as
needed too.

2021-05-19  Christophe Lyon  <christophe.lyon@linaro.org>

	PR target/99977
	gcc/testsuite/
	* gcc.target/arm/pr99977.c: Require arm_arch_v8m_base.