implement mc asmparser support for '.', which gets the

current PC.  rdar://7834775

We now produce an identical .o file compared to the cctools
assembler for something like this:

_f0:
L0:
        jmp L1
        .long . - L0
L1:
        jmp A
        .long . - L1

        .zerofill __DATA,_bss,A,0

llvm-svn: 101227