Summary:
* Throw an error when cross-compiling without a target definition.
  When cross compiling via LLVM, a target 'datalayout' and 'triple' must
  be defined or LLVM will generate code for the compile host instead of
  the compile target.

* Add aarch64-unknown-linux-gnu target.
  The datalayout and triple lines were found by using clang to compile a
  small C program and -emit-llvm to get the LLVM IR output.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>

Test Plan: validate

Reviewers: rwbarton, carter, hvr, bgamari, austin

Reviewed By: austin

Subscribers: carter, thomie, garious

Differential Revision: https://phabricator.haskell.org/D585

GHC Trac Issues: #9895