x86/alternative: Try inline spectre_v2=retpoline,amd

Try and replace retpoline thunk calls with:

  LFENCE
  CALL    *%\reg

for spectre_v2=retpoline,amd.

Specifically, the sequence above is 5 bytes for the low 8 registers,
but 6 bytes for the high 8 registers. This means that unless the
compilers prefix stuff the call with higher registers this replacement
will fail.

Luckily GCC strongly favours RAX for the indirect calls and most (95%+
for defconfig-x86_64) will be converted. OTOH clang strongly favours
R11 and almost nothing gets converted.

Note: it will also generate a correct replacement for the Jcc.d32
case, except unless the compilers start to prefix stuff that, it'll
never fit. Specifically:

  Jncc.d8 1f
  LFENCE
  JMP     *%\reg
1:

is 7-8 bytes long, where the original instruction in unpadded form is
only 6 bytes.

Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Borislav Petkov <[email protected]>
Acked-by: Josh Poimboeuf <[email protected]>
Tested-by: Alexei Starovoitov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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