[mach-o] Let darwin driver infer arch from .o files if -arch not used.

Mach-O has a "fat" (or "universal") variant where the same contents built for
different architectures are concatenated into one file with a table-of-contents
header at the start.  But this leaves a dilemma for the linker - which
architecture to use.

Normally, the linker command line -arch is used to force which slice of any fat
files are used.  The clang compiler always passes -arch to the linker when
invoking it.  But some Makefiles invoke the linker directly and don’t specify
the -arch option.  For those cases, the linker scans all input files in command
line order and finds the first non-fat object file.  Whatever architecture it
is becomes the architecture for the link.

llvm-svn: 217189