testsuite: require cortex-a53 in reg_equal_test.c test

The original purpose of the test is to verify that, when the backend
emits a mov-pair sequence, the sequence carries a REG_EQUAL note for the
final constant value.  Make that precondition explicit instead of
depending on target-specific tuning behavior, by forcing the test to
a CPU with the required properties.

gcc/testsuite/ChangeLog:

	* gcc.target/arm/reg_equal_test.c: Add require effective target
	cortex-a53 for test.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
(cherry picked from commit 8d16b075796e77e2160bee01678b35fc0568dc36)