runtime: set procid on Windows

runtime: set procid on Windows

The runtime-gdb.py script needs procid to be set in order to
map a goroutine ID with an OS thread. The Go runtime is not currently
setting that variable on Windows, so TestGdbPython (and friends) can't
succeed.

This CL initializes procid and unskips gdb tests on Windows.

Fixes #22687
Updates #21380
Updates #22021

Change-Id: Icd1d9fc1764669ed1bf04f53d17fadfd24ac3f30
Reviewed-on: https://go-review.googlesource.com/c/go/+/470596
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>