Implement PR512:

This patch adds a -post-link-opts option to llvm-ld which allows an arbitrary
program to optimize bytecode after linking. The program is passed two file
names. The first is the input (linked bytecode) the second is where it must
place its output (presumably after optimizing). If the output file is bytecode,
it is used as a substitute for the input. This will allow things like poolalloc
to be written as a separate program instead of a loadable module or built into
LLVM.

llvm-svn: 24893