mk: Try to fix nightlies again

Looks like the real bug on nightlies is that the `llvm-pass` run-make test is
not actually getting the value of `LLVM_CXXFLAGS` correct. Namely, it's blank!
Now the only change #33093 which actually affected this is that the argument
`$(LLVM_CXXFLAGS_$(2))` was moved up from a makefile rule into the definition of
a variable. Sounds innocuous?

Turns out the variable this was moved into is defined with `:=`, which means
that it's not recursively expanded, which basically means that it's expanded
immediately. Unfortunately part of this expansion involves running
`llvm-config`, which doesn't exist at the start of distcheck build!

This didn't show up on the bots because they run `make` *then* `make check`, and
the first step builds llvm-config so the next time `make` is loaded everything
is available. The distcheck bots, however, run just a plain `distcheck` so
`make` doesn't exist ahead of time. You can see this in action where the
distcheck bots start out with a bunch of "llvm-config not found" error messages.

This commit just changes a few variables to be defined with `=` which
essentially means they're lazily expanded. I did not run a full distcheck
locally, but this makes the initial "llvm-config not found" error messages go
away so I suspect that this is the fix.

Closes #33379

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