ghc-cabal: Use correct name of linker flags env variable
ghc-cabal: Use correct name of linker flags env variable

Currently passing the `CONF_LD_LINKER_OPTS_STAGE0` environment
variable to `configure` is broken due to this naming inconsistency.

Test Plan: Try passing `CONF_LD_LINKER_OPTS_STAGE0` to `configure`.
Look at resulting stage0 ghc invocation.

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2672

olsner/ghc,shlevy/ghc,ezyang/ghc,mettekou/ghc,sdiehl/ghc,mettekou/ghc,olsner/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,mettekou/ghc,ezyang/ghc,mettekou/ghc,ezyang/ghc,shlevy/ghc,olsner/ghc,shlevy/ghc,mettekou/ghc,shlevy/ghc,olsner/ghc,shlevy/ghc,sdiehl/ghc,olsner/ghc,olsner/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,mettekou/ghc,olsner/ghc,mettekou/ghc