x86: guard against straight-line speculation past RET

x86: guard against straight-line speculation past RET

Under certain conditions CPUs can speculate into the instruction stream
past a RET instruction. Guard against this just like 3b7dab93f240
("x86/spec-ctrl: Protect against CALL/JMP straight-line speculation")
did - by inserting an "INT $3" insn. It's merely the mechanics of how to
achieve this that differ: A set of macros gets introduced to post-
process RET insns issued by the compiler (or living in assembly files).

Unfortunately for clang this requires further features their built-in
assembler doesn't support: We need to be able to override insn mnemonics
produced by the compiler (which may be impossible, if internally
assembly mnemonics never get generated).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>