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.

gcc/testsuite/ChangeLog

+2020-01-27  Richard Sandiford  <richard.sandiford@arm.com>
+
+	PR testsuite/71727
+	* gcc.target/aarch64/pr71727.c: Add -fcommon.
+
 2020-01-27  David Malcolm  <dmalcolm@redhat.com>
 
 	* gcc.dg/analyzer/sigsetjmp-5.c: New test.

gcc/testsuite/gcc.target/aarch64/pr71727.c

 /* { dg-do compile } */
-/* { dg-options "-mstrict-align -O3" } */
+/* { dg-options "-mstrict-align -O3 -fcommon" } */
 
 struct test_struct_s
 {