[RuntimeDyld] Add a framework for testing relocation logic in RuntimeDyld.

This patch adds a "-verify" mode to the llvm-rtdyld utility. In verify mode,
llvm-rtdyld will test supplied expressions against the linked program images
that it creates in memory. This scheme can be used to verify the correctness
of the relocation logic applied by RuntimeDyld.

The expressions to test will be read out of files passed via the -check option
(there may be more than one of these). Expressions to check are extracted from
lines of the form:
# rtdyld-check: <expression>

This system is designed to fit the llvm-lit regression test workflow. It is
format and target agnostic, and supports verification of images linked for
remote targets. The expression language is defined in
llvm/include/llvm/RuntimeDyldChecker.h . Examples can be found in
test/ExecutionEngine/RuntimeDyld.