aarch64: Fix pr71727.c failure

This test started failing after the switch to -fno-common because we can
now force the array to be aligned to 16 bytes, which in turn lets us use
SIMD accesses.  Locally restoring -fcommon seems the most faithful to
the original PR.

2020-01-27  Richard Sandiford  <richard.sandiford@arm.com>

gcc/testsuite/
	PR testsuite/71727
	* gcc.target/aarch64/pr71727.c: Add -fcommon.