x86/retpoline: split retpoline compiler support into separate option

x86/retpoline: split retpoline compiler support into separate option

Keep the previous option as a way to signal generic retpoline support
regardless of the underlying compiler, while introducing a new
CC_HAS_INDIRECT_THUNK that signals whether the underlying compiler
supports retpoline.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: e245bc154300b5d0367b64e8b937c9d1da508ad3
master date: 2022-02-21 18:17:56 +0000