Backward references in assembly absolute expressions

I'm also interested in this area as we're adapting our VAX Macro32 'assembler'
to LLVM x86-64 at the MCInst level. These sorts of things happen all the time
for me so having some underlying support is helpful. We have our own
parser/middle-end of course.

We'll also be adding some out-of-tree support for non-absolute expressions
as well for OpenVMS. We have ELF relocation extensions for Itanium to let
that happen.

For example,

.long external_name1 - external_name2

OpenVMS has had this since the beginning (1970s) and we can't seem to shake it.
I'm not sure anybody else would want this so we have to keep it local.