Fix JITLoaderGDB for 64-bit host and 32-bit target

There are target pointer members in struct jit_code_entry and jit_descriptor.

Data layout of those structures should be decided by target, not host.

This fixes JITLoaderGDB for 64-bit host and 32-bit target.

See http://reviews.llvm.org/D5339 for more details.

Change by Tong Shen.