Fixes the ARM build

Fixes the ARM build

Summary:
CodeGen.Platform.hs was changed with the following diff:

   -#endif
    globalRegMaybe _                        = Nothing
   +#elif MACHREGS_NO_REGS
   +globalRegMaybe _ = Nothing
   +#else
   +globalRegMaybe = panic "globalRegMaybe not defined for this platform"
   +#endif

which causes globalRegMaybe ot panic for arch ARM.

This patch ensures globalRegMaybe is not called on ARM.

Signed-off-by: Moritz Angermann <moritz@lichtzwerge.de>
Test Plan: Building arm cross-compiler (e.g. --target=arm-apple-darwin10)

Reviewers: hvr, ezyang, simonmar, rwbarton, austin

Reviewed By: austin

Subscribers: dterei, bgamari, simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D208

GHC Trac Issues: #9593