lang/ghc: use CONFIGURE_ENV instead of BINARY_ALIAS on arm

GHC saves the llvm command name (llc, opt, clang) found at compile time in its
settings file.
Using BINARY_ALIAS allows lang/ghc to build but breaks
ghc ports (devel/hs-cabal-install for example)
GHC configure script accepts now to define LLC, OPT, CLANG as
environment variable, switch to it.

<no location info>: error:

Warning: Couldn't figure out LLVM version!
         Make sure you have installed LLVM 9

<no location info>: error: ghc: could not execute: opt-9.0

PR: 252166, 253367
Approved by: portmgr (tier-2 blanket)