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

(cherry picked from commit 97505ad7b23f5e43e1e1a69c159c2c7054f600e7)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc