compiler: start deprecating cmmToRawCmmHook

cmmToRawCmmHook was added 4 years ago in
d561c8f6. Its only user is the
Asterius project, which has been archived and deprecated in favor of
the ghc wasm backend. This patch starts deprecating cmmToRawCmmHook by
placing a DEPRECATED pragma, and actual removal shall happen in a
future GHC major release if no issue to oppose the deprecation has
been raised in the meantime.