LTO: Add -fcf-protection=check

LTO: Add -fcf-protection=check

Mixing -fcf-protection and -fcf-protection=none objects are allowed.
Linker just merges -fcf-protection values from all input objects.

Add -fcf-protection=check for the final link with LTO.  An error is
issued if LTO object files are compiled with different -fcf-protection
values.  Otherwise, -fcf-protection=check is ignored at the compile
time.  Without explicit -fcf-protection at link time, -fcf-protection
values from LTO object files are merged at the final link.

gcc/

	PR bootstrap/96203
	* common.opt: Add -fcf-protection=check.
	* flag-types.h (cf_protection_level): Add CF_CHECK.
	* lto-wrapper.c (merge_and_complain): Issue an error for
	mismatching -fcf-protection values with -fcf-protection=check.
	Otherwise, merge -fcf-protection values.
	* doc/invoke.texi: Document -fcf-protection=check.

gcc/testsuite/

	PR bootstrap/96203
	* gcc.target/i386/pr96203-1.c: New test.
	* gcc.target/i386/pr96203-2.c: Likewise.