Make the JIT code emitter properly retry and ask for more memory when it runs

out of memory, and also make the default memory manager allocate more memory
when it runs out.

Also, switch function stubs and global data over to using the BumpPtrAllocator.

This makes it so the JIT no longer mmaps (or the equivalent on Windows) 16 MB
of memory, and instead allocates in 512K slabs.  I suspect this size could go
lower, especially on embedded platforms, now that more slabs can be allocated.

llvm-svn: 76828