[MC] Support labels as offsets in .reloc directive

Currently, expressions like

  .reloc 1f, R_MIPS_JALR, foo
  1: nop

are not allowed, ie. an offset in .reloc can only be absolute value.
This patch adds support for labels as offsets.
If offset is a forward declared label, MCObjectStreamer keeps the fixup locally
and adds it to the fixups vector after the label (and its offset) is defined.
label+number is not supported yet.

Differential revision: https://reviews.llvm.org/D53990

llvm-svn: 347397