Alec Theriault authored Nov 22, 2018 and Ben Gamari committed Nov 22, 2018

The path to the 'include' subdirectory of 'rts' includes a folder that
whose name is generated by Cabal and mentiones the architecture and OS.
For example:

    _build/stage1/lib/x86_64-osx-ghc-8.7.20181120/rts-1.0/include

Hadrian needs to be aware that Cabal renders architectures and OSes in
a slightly different way than GHC. There is already symmetric logic in
Cabal (for working with GHC environment files, which follow GHC's naming
conventions).

Test Plan: ./hadrian/build.sh -c "binary-dist" # on mac

Reviewers: snowleopard, alpmestan, bgamari

Reviewed By: snowleopard

Subscribers: rwbarton, carter

GHC Trac Issues: #15922

Differential Revision: https://phabricator.haskell.org/D5361