Suppress AMP warnings with -XNoImplicitPrelude (fixed Trac #8320)

The AMP warnings caused 'base' to be loaded even when we were
compiling 'ghc-prim'.  That is bad, bad, bad.  We got a very
obscure message
   attempting to use module ‛ghc-prim:GHC.Types’
     (libraries/ghc-prim/./GHC/Types.hs) which is not loaded

See Note [Home module load error] in LoadIface