This CL allows ConstantRelocatables in to Subzero
have symbolic constants. A symbolic constant is an
assembly label whose value is not known during
lowering, but it is well defined during code
emission.

For example, the following code is now possible in Subzero:

foo:
  push $foo.bar
  jmp target
  nop
  nop
foo.bar:
  ...

BUG=
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1651163002 .