x86: Add -mindirect-branch-cs-prefix

x86: Add -mindirect-branch-cs-prefix

Add -mindirect-branch-cs-prefix to add CS prefix to call and jmp to
indirect thunk with branch target in r8-r15 registers so that the call
and jmp instruction length is 6 bytes to allow them to be replaced with
"lfence; call *%r8-r15" or "lfence; jmp *%r8-r15" at run-time.

gcc/

	PR target/102952
	* config/i386/i386.c (ix86_output_jmp_thunk_or_indirect): Emit
	CS prefix for -mindirect-branch-cs-prefix.
	(ix86_output_indirect_branch_via_reg): Likewise.
	* config/i386/i386.opt: Add -mindirect-branch-cs-prefix.
	* doc/invoke.texi: Document -mindirect-branch-cs-prefix.

gcc/testsuite/

	PR target/102952
	* gcc.target/i386/indirect-thunk-cs-prefix-1.c: New test.
	* gcc.target/i386/indirect-thunk-cs-prefix-2.c: Likewise.