[lit] Allow passthrough of some QEMU_* environment variables to lit (#111373)

This is an alternate implementation of a patch proposed by @preames in
<https://reviews.llvm.org/D128840>. As noted there, when running
non-native binaries with binfmt_misc and qemu-user you typically need to
set some environment variables (at least, QEMU_LD_PREFIX), but lit
strips them by default. This patch adds what I think are the two main
ones to the list of those that aren't stripped. It does so in a place
that applies to all lit test suites (rather than just LLVM's), and as
can be seen from the other env vars in `pass_vars` I think there's
already plenty of precedent for passing through environment variables
known to be potentially useful to LLVM developers.

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