Fix for clang-upc issue #57, fix UPC link script off/on.

CMakeLists.txt
autoconf/configure.ac
include/llvm/Config/config.h.cmake
include/llvm/Config/config.h.in
	Check and set the usage of the UPC link script.
configure
	Regenerate.
include/llvm/MC/MCObjectFileInfo.h
lib/MC/MCObjectFileInfo.cpp
	Add UPC shared section and initialize it only if linker
        script is not used.  Otherwise, linker script defines it.