Add the -arch flag support to llvm-nm to select the slice out of a Mach-O

Add the -arch flag support 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@212054 91177308-0d34-0410-b5e6-96231b3b80d8