This adds a Darwin x86_64 relocation encoding for a subtraction expression

where both symbols are "local", that is non-external symbols, and there is
no "base" for the symbols used in the expression, that is the section has
no non-temporary symbols.  This case looks like this:

% cat local_reloc_A-B.s
	.long 0
LB:	.long 1
	.long LA - LB - 4
	.long 2
LA:	.long 3

which llvm-mc will not encode without this patch, generates a "unsupported
local relocations in difference" error, but the Darwin assembler will
encode with relocation entries like this:

% otool -rv a.out l.out 
a.out:
Relocation information (__TEXT,__text) 2 entries
address  pcrel length extern type    scattered symbolnum/value
00000008 False long   False  SUB     False     1 (__TEXT,__text)
00000008 False long   False  UNSIGND False     1 (__TEXT,__text)

which is very similar to what is encoded when the symbols don't have the
leading 'L' and they are not temporary symbols.  Which llvm-mc and the
Darwin assembler will encoded like this:

Relocation information (__TEXT,__text) 2 entries
address  pcrel length extern type    scattered symbolnum/value
00000008 False long   True   SUB     False     B
00000008 False long   True   UNSIGND False     A

This is the missing relocation encoding needed to allow the Mach-O x86
Dwarf file and line table to be emitted.  So this patch also removes the
TODO from the if() statement in MCMachOStreamer::Finish() that didn't 
call MCDwarfFileTable::Emit() for 64-bit targets.

llvm-svn: 115389