x86: Allow -fcf-protection with external thunk

Allow -fcf-protection with external thunk since the external thunk can be
made compatible with -fcf-protection.

gcc/

	PR target/93654
	* config/i386/i386-options.c (ix86_set_indirect_branch_type):
	Allow -fcf-protection with -mindirect-branch=thunk-extern and
	-mfunction-return=thunk-extern.
	* doc/invoke.texi: Update notes for -fcf-protection=branch with
	-mindirect-branch=thunk-extern and -mindirect-return=thunk-extern.

gcc/testsuite/

	PR target/93654
	* gcc.target/i386/pr93654.c: New test.