testsuite: Add tests for C23 __STDC_VERSION__

Add some tests for the value of __STDC_VERSION__ in C23 mode.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

	* gcc.dg/c23-version-1.c, gcc.dg/c23-version-2.c,
	gcc.dg/gnu23-version-1.c: New tests.