Introduce selftest::locate_file

gcc/ChangeLog:
	* Makefile.in (SELFTEST_FLAGS): Add path argument to -fself-test.
	(s-selftest): Add dependency on the selftests data directory.
	* common.opt (fself-test): Rename to...
	(fself-test=): ...this, documenting the meaning of the argument.
	* selftest-run-tests.c (along): Likewise.
	* selftest-run-tests.c: Include "options.h".
	(selftest::run_tests): Initialize selftest::path_to_selftest_files
	from flag_self_test.
	* selftest.c (selftest::path_to_selftest_files): New global.
	(selftest::locate_file): New function.
	(selftest::test_locate_file): New function.
	(selftest_c_tests): Likewise.
	(selftest::selftest_c_tests): Call test_locate_file.
	* selftest.h (selftest::locate_file): New decl.
	(selftest::path_to_selftest_files): New decl.

gcc/testsuite/ChangeLog:
	PR target/78213
	* gcc.dg/cpp/pr71591.c: Add a fake value for the argument of
	-fself-test.
	* gcc.dg/pr78213.c: Disable this test.
	* selftests/example.txt: New file.

From-SVN: r243681