[llvm-jitlink] Add optional slab allocator for testing locality optimizations.

[llvm-jitlink] Add optional slab allocator for testing locality optimizations.

The llvm-jitlink utility now accepts a '-slab-allocate <size>' option. If given,
llvm-jitlink will use a slab-based memory manager rather than the default
InProcessMemoryManager. Using a slab allocator will allow reliable testing of
future locality based optimizations (e.g. PLT and GOT elimination) in JITLink.

The <size> argument is a number, optionally followed by a units specifier (Kb,
Mb, or Gb). If the units are not given then the number is assumed to be in Kb.

llvm-svn: 371244