x86: Document -mcet-switch

x86: Document -mcet-switch

When -fcf-protection=branch is used, the compiler will generate jump
tables for switch statements where the indirect jump is prefixed with
the NOTRACK prefix, so it can jump to non-ENDBR targets.  Since the
indirect jump targets are generated by the compiler and stored in
read-only memory, this does not result in a direct loss of hardening.
But if the jump table index is attacker-controlled, the indirect jump
may not be constrained by CET.

Document -mcet-switch to generate jump tables for switch statements with
ENDBR and skip the NOTRACK prefix for indirect jump.  This option should
be used when the NOTRACK prefix is disabled.

	PR target/104816
	* config/i386/i386.opt: Remove Undocumented.
	* doc/invoke.texi: Document -mcet-switch.