gold: Compile common tests with -fcommon

gold: Compile common tests with -fcommon

Since GCC 10 defaults to -fno-common, add -fcommon to common tests to
force common behavior.

	PR gold/25904
	* testsuite/Makefile.am (COMMON_TEST_C_CFLAGS): New.
	(common_test_1.o): New rule.
	(common_test_2.o): Likewise.
	(common_test_3.o): Likewise.
	(plugin_common_test_1.o): Likewise.
	(plugin_common_test_2.o): Likewise.
	(common_test_1_v1.o): Likewise.
	(common_test_1_v2.o): Likewise.
	(common_test_2_pic.o): Compile with $(COMMON_TEST_C_CFLAGS).
	(common_test_3_pic.o): Likewise.
	* testsuite/Makefile.in: Regenerated.