gallivm: Properly update for removal of JITMemoryManager in LLVM 3.6.

gallivm: Properly update for removal of JITMemoryManager in LLVM 3.6.

JITMemoryManager was removed in LLVM 3.6, and replaced by its base class
RTDyldMemoryManager.

This change fixes our JIT memory managers specializations to derive from
RTDyldMemoryManager in LLVM 3.6 instead of JITMemoryManager.

This enables llvmpipe to run with LLVM 3.6.

However, lp_free_generated_code is basically a no-op because there are
not enough hook points in RTDyldMemoryManager to track and free the code
of a module.  In other words, with MCJIT, code once created, stays
forever allocated until process destruction.  This is not speicfic to
LLVM 3.6 -- it will happen whenever MCJIT is used regardless of version.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>