arm: fix testsuite failure of reg_equal_test.c [PR101755]

arm: fix testsuite failure of reg_equal_test.c [PR101755]

The test failure in PR101755 is due to the gimple optimizers getting
smarter.  But really we are just testing that RTL expansion is doing
the right thing and annotating a constant accordingly.  So rework the
test to use GIMPLE input and simplify the code entirely.  Also, this
test only ever worked on architecture versions with the MOVW/MOVT
instructions, so check for this before running.

gcc/testsuite/ChangeLog:

	PR target/101755
	* gcc.target/arm/reg_equal_test.c: Convert to gimple test.  Restrict
	to architectures with MOVW/MOVT.