Before this commit, GHC only supported LLVM 3.6. Now it only supports
LLVM 3.7 which was released in August 2015. LLVM version 3.6 and earlier
do not work on AArch64/Arm64, but 3.7 does.

Also:
* Add CC_Ghc constructor to LlvmCallConvention.
* Replace `maxSupportLlvmVersion`/`minSupportLlvmVersion` with
  a single `supportedLlvmVersion` variable.
* Get `supportedLlvmVersion` from version specified in configure.ac.
* Drop llvmVersion field from DynFlags (no longer needed because only
  one version is supported).

Test Plan: Validate on x86_64 and arm

Reviewers: bgamari, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1320

GHC Trac Issues: #10953