This PR propagates "value labels" all the way from CLIF to DWARF
metadata on the emitted machine code. The key idea is as follows:

- Translate value-label metadata on the input into "value_label"
  pseudo-instructions when lowering into VCode. These
  pseudo-instructions take a register as input, denote a value label,
  and semantically are like a "move into value label" -- i.e., they
  update the current value (as seen by debugging tools) of the given
  local. These pseudo-instructions emit no machine code.

- Perform a dataflow analysis *at the machine-code level*, tracking
  value-labels that propagate into registers and into [SP+constant]
  stack storage. This is a forward dataflow fixpoint analysis where each
  storage location can contain a *set* of value labels, and each value
  label can reside in a *set* of storage locations. (Meet function is
  pairwise intersection by storage location.)

  This analysis traces value labels symbolically through loads and
  stores and reg-to-reg moves, so it will naturally handle spills and
  reloads without knowing anything special about them.

- When this analysis converges, we have, at each machine-code offset, a
  mapping from value labels to some number of storage locations; for
  each offset for each label, we choose the best location (prefer
  registers). Note that we can choose any location, as the symbolic
  dataflow analysis is sound and guarantees that the value at the
  value_label instruction propagates to all of the named locations.

- Then we can convert this mapping into a format that the DWARF
  generation code (wasmtime's debug crate) can use.

This PR also adds the new-backend variant to the gdb tests on CI.

I had missed that the CI config didn't actually run the tests, because
(I think) `matrix.target` is not set by default (?). All of our hosts
are native x86-64, so we can just gate on OS (Ubuntu) instead.

I also discovered that while I had been testing with the gdb tests
locally, when *all* `debug::*` tests are run, there are two that do not
pass on the new backend because of specific differences in compiled
code. One is a value-lifetime issue (the value is "optimized out" at the
point the breakpoint is set) and the other has to do with basic-block
order (it is trying to match against hardcoded machine-code offsets
which have changed).