CMake: Stop using LLVM's custom parse_arguments. NFC

Summary:
Use CMake's cmake_parse_arguments() instead.
It's called in a slightly different way, but supports all our use cases.
It's in CMake 2.8.8, which is our minimum supported version.

CMake 3.0 doc (roughly the same. No direct link to 2.8.8 doc):
http://www.cmake.org/cmake/help/v3.0/module/CMakeParseArguments.html?highlight=cmake_parse_arguments

Since I was already changing these calls, I changed ARCH and LIB into
ARCHS and LIBS to make it more clear that they're lists of arguments.

Reviewers: eugenis, samsonov, beanz

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10529

llvm-svn: 240120