testsuite: Adjust gcc.dg/cpp/if-2.c for 16-bit targets

Jeff reported this testcase newly FAILs on 16-bit targets, the following
patch adjusts the expected diagnostics for that case.

2023-11-15  Jakub Jelinek  <jakub@redhat.com>

	* gcc.dg/cpp/if-2.c: Adjust expected diagnostics for 16-bit targets.