x86: Add -mharden-sls=[none|all|return|indirect-branch]

x86: Add -mharden-sls=[none|all|return|indirect-branch]

Add -mharden-sls= to mitigate against straight line speculation (SLS)
for function return and indirect branch by adding an INT3 instruction
after function return and indirect branch.

gcc/

	PR target/102952
	* config/i386/i386-opts.h (harden_sls): New enum.
	* config/i386/i386.c (output_indirect_thunk): Mitigate against
	SLS for function return.
	(ix86_output_function_return): Likewise.
	(ix86_output_jmp_thunk_or_indirect): Mitigate against indirect
	branch.
	(ix86_output_indirect_jmp): Likewise.
	(ix86_output_call_insn): Likewise.
	* config/i386/i386.opt: Add -mharden-sls=.
	* doc/invoke.texi: Document -mharden-sls=.

gcc/testsuite/

	PR target/102952
	* gcc.target/i386/harden-sls-1.c: New test.
	* gcc.target/i386/harden-sls-2.c: Likewise.
	* gcc.target/i386/harden-sls-3.c: Likewise.
	* gcc.target/i386/harden-sls-4.c: Likewise.
	* gcc.target/i386/harden-sls-5.c: Likewise.