[Patch] OpenMP: Add omp_default_device named constant [PR119677]

[Tobias Burnus]

This is a collateral patch from working on an update for the
implementation status, namely on adding the TR14 (OpenMP 6.1)
additions to https://gcc.gnu.org/onlinedocs/libgomp/OpenMP-Implementation-Status.html

Actually, I missed this feature myself - most of the time
when calling API routines that take a device number as argument;
'0' feels wrong – and 'omp_get_default_device()' is somewhat
lengthy. In any case, this patch adds this extremely low
hanging fruit.

Comments before I commit it?

Tobias

PS: This only adds omp_default_device; the PR also mentions
some cleanup tasks, but that's deferred to later.