Refactor TargetOptions initialization into a single place.

The same code (~20 lines) for initializing a TargetOptions object from CodeGen
cmdline flags is duplicated 4 times in 4 different tools. This patch moves it
into a utility function.

Since the CodeGen/CommandFlags.h file defines cl::opt flags in a header, it's
a bit of a touchy situation because we should only link them into tools. So this
patch puts the init function in the header.