[PR102706] [testsuite] -Wno-stringop-overflow vs Warray-bounds

[PR102706] [testsuite] -Wno-stringop-overflow vs Warray-bounds

The bogus Wstringop-overflow warnings conditionally issued for
Warray-bounds-48.c and -Wzero-length-array-bounds-2.c are expected
under conditions that depend on the availability of certain vector
patterns, but that don't seem to model the conditions under which the
warnings are expected.

On riscv64-elf and arm-eabi/-mcpu=cortex-r5, for example, though the
Warray-bounds-48.c condition passes, we don't issue warnings.  On
riscv64-elf, we decide not to vectorize the assignments; on cortex-r5,
we do vectorize pairs of assignments, but that doesn't yield the
expected warning, even though assignments that should trigger the
bogus warning are vectorized and associated with the earlier line
where the bogus warning would be expected.

On riscv64, for Wzero-length-array-bounds-2.c, we issue the expected
warning in test_C_global_buf, but we also issue a warning for
test_C_local_buf under the same conditions, that would be expected on
other platforms but that is not issued on them.  On
arm-eabi/-mcpu=cortex-r5, the condition passes so we'd expect the
warning in both functions, but we don't warn on either.

Instead of further extending the effective target tests, introduced to
temporarily tolerate these expected bogus warnings, so as to capture
the vectorizer analyses that lead to the mismatched decisions, I'm
disabling the undesired warnings for these two tests.

for  gcc/testsuite/ChangeLog

	PR tree-optimization/102706
	* gcc.dg/Warray-bounds-48.c: Disable -Wstringop-overflow.
	* gcc.dg/Wzero-length-array-bounds-2.c: Likewise.