[ELF] - Linkerscript: implemented += operator.

Sometimes += is used to move the location counter.
Example from the wild is:

.dbg_excpt _DBG_EXCPT_ADDR (NOLOAD) :
{
  . += (DEFINED (_DEBUGGER) ? 0x8 : 0x0);
https://github.com/chipKIT32/pic32-Arduino-USB-Bootloader-original/blob/master/boot-linkerscript.ld

Patch implements it and opens way for others type of assignments (-= *= etc), though I think only += is
actual to support.

Differential revision: https://reviews.llvm.org/D22916

llvm-svn: 277035