runtime: tricky replacements of _g_ in proc.go

This manually replaces uses of _g_ that cannot be trivially switched to
gp since there is another gp variable in scope.

Most of these functions only use the current g to reach the m, so this
helps with clarity by switching all accesses directly to an mp variable.

Change-Id: I96a4fc1c32470a7f3d12ddec9f147c2743210e71
Reviewed-on: https://go-review.googlesource.com/c/go/+/418577


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