Fix checking -mibt and -mshstk options for control flow protection

Fix checking -mibt and -mshstk options for control flow protection

	PR target/84145
	* config/i386/i386.c: Reimplement the check of possible options
	-mibt/-mshstk conbination. Change error messages.
	* doc/invoke.texi: Fix a typo: remove extra '='.
	* c-c++-common/fcf-protection-1.c: Change a compared message.
	* c-c++-common/fcf-protection-2.c: Likewise.
	* c-c++-common/fcf-protection-3.c: Likewise.
	* c-c++-common/fcf-protection-5.c: Likewise.
	* c-c++-common/fcf-protection-6.c: New test.
	* c-c++-common/fcf-protection-7.c: Likewise.

From-SVN: r257414