I intent to commit this tomorrow, unless there are comments.

It does as it says (see commit log): It initializes default-device-var
to the value using the algorithm described in OpenMP 5.2, which
depends on whether OMP_TARGET_OFFLOAD=mandatory was set.

NOTE: With -foffload=disable there is no binary code but still
devices get found - such that default-device-var == 0 (= first
nonhost device). Thus, in that case, libgomp runs the code on that
device but as no binary data is available, host fallback is used.
(Even if there would be executable code for another device on
the system.)
With mandatory, this unintended host fallback is detected and an
error is diagnosed. One can argue whether keeping the devices
makes sense (e.g. because in a dynamic library device code will
be loaded later) or not (don't list if no code is available).

Note that TR11 (future OpenMP 6.0) extends OMP_DEFAULT_DEVICE and
adds OMP_AVAILABLE_DEVICES which permit a finer-grained control about
the device, including OMP_DEFAULT_DEVICE=initial (and 'invalid') which
the current scheme does not permit. (Well, there is
OMP_TARGET_OFFLOAD=disabled, but that's a too big hammer.)

Tobias

PS:  DejaGNU testing was done without offloading configured
and with remote testing on a system having an offload device,
which which does not support setting environment variables.
Manual testing was done with offloading enabled and depending
on the testcase, running on a system with and/or without offloading
hardware.
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