x86,retpoline: Optimize patch_retpoline()

Currently the very common retpoline: "CS CALL __x86_indirect_thunk_r11"
is transformed into "CALL *R11; NOP3" for eIBRS/BHI_NO parts.

Similarly, paranoid fineibt has: "CALL *R11; NOP".

Recognise that CS stuffing can avoid the extra NOP. However, due to
prefix decode penalties, make sure to not emit too many CS prefixes.
Notably: "CS CALL __x86_indirect_thunk_rax" must not become "CS CS CS
CS CALL *RAX". Prefix decode penalties are typically many more cycles
than decoding an extra NOP.

Additionally, if the retpoline is a tail-call, the "JMP *%\reg" should
be followed by INT3 for straight-line-speculation mitigation, since
emit_indirect() now has a length argument, move this into
emit_indirect() such that other users (paranoid-fineibt) also do this.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250902104627.GM4068168@noisy.programming.kicks-ass.net

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