runtime: trivial replacements of _g_ in proc.go

Generated with global replace: 's/_g_/gp/g'.

Change-Id: Ia91606a0a8a5773be7c6e5152160510ae9bb221e
Reviewed-on: https://go-review.googlesource.com/c/go/+/418584


Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>