Add the -arch flag support to llvm-size like what was done to llvm-nm

Add the -arch flag support to llvm-size like what was done to llvm-nm
to select the slice out of a Mach-O universal file.  This also includes
support for -arch all, selecting the host architecture by default from
a universal file and checking if -arch is used with a standard Mach-O
it matches that architecture.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212108 91177308-0d34-0410-b5e6-96231b3b80d8