clover/llvm: Clean up compile_native().

clover/llvm: Clean up compile_native().

This switches compile_native() to the C++ API (which the rest of this
file makes use of anyway so there is little benefit from using the C
API), what should get rid of an amount of boilerplate and fix a leak
of the TargetMachine object in the error path.

v2: Additional fixes for LLVM 3.6.
v3: Update for the latest LLVM SVN changes.

Reviewed-by: Serge Martin <edb+mesa@sigluy.net>
Tested-by: Jan Vesely <jan.vesely@rutgers.edu>