llvm-c: Make target initializer functions external functions in lib.

Making them proper functions defined in the (shared)lib instead of
static inlines defined in the header files makes it possible to
actually distribute a binary compiled against the shared library
without having to worry about getting undefined symbol errors when
calling e.g LLVMInitializeAllTargetInfos because the shared library on
the other system was compiled with different targets.

Differential Revision: http://llvm-reviews.chandlerc.com/D1714