[libstdc++] [testsuite] defer to check_vect_support* [PR115454]

[libstdc++] [testsuite] defer to check_vect_support* [PR115454]

The newly-added testcase overrides the default dg-do action set by
check_vect_support_and_set_flags (in libstdc++-dg/conformance.exp), so
it attempts to run the test even if runtime vector support is not
available.

Remove the explicit dg-do directive, so that the default is honored,
and the test is run if vector support is found, and only compiled
otherwise.

for  libstdc++-v3/ChangeLog

	PR libstdc++/115454
	* testsuite/experimental/simd/pr115454_find_last_set.cc: Defer
	to check_vect_support_and_set_flags's default dg-do action.