llvm-mc/Mach-O: Improve symbol table support:

llvm-mc/Mach-O: Improve symbol table support:
 - Honor .globl.

 - Set symbol type and section correctly ('nm' now works), and order symbols
   appropriately.

 - Take care to the string table so that the .o matches 'as' exactly (for ease
   of testing).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79740 91177308-0d34-0410-b5e6-96231b3b80d8