Rewrite JIT handling of GlobalVariables so they

are allocated in the same buffer as the code,
jump tables, etc.

The default JIT memory manager does not handle buffer
overflow well.  I didn't introduce this and I'm not
attempting to fix it here, but it is more likely to
be hit now since we're putting more stuff in the
buffer.  This affects one test that I know of so far,
MultiSource/Benchmarks/NPB-serial/is.

llvm-svn: 54442