testsuite: arm: Use effective-target for pr84556.cc test

testsuite: arm: Use effective-target for pr84556.cc test

Using "dg-do run" with a selector overrides the default selector set by
vect.exp that picks between "dg-do run" and "dg-do compile" based on the
target's support for simd operations for Arm targets.
The actual selection of default operation is performed in
check_vect_support_and_set_flags.

gcc/testsuite/ChangeLog:

	* g++.dg/vect/pr84556.cc: Change from "dg-do run" with selector
	to instead use dg-require-effective-target with the same
	selector.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>