runtime: tricky replacements of _g_ in mgc.go
runtime: tricky replacements of _g_ in mgc.go

getg() is used primarily for gp.m, so use mp. gp.m.curg is also used,
name that curgp.

Change-Id: I9db2dc4adce2b17d6b41c60d4f2dfbc9aa41fac1
Reviewed-on: https://go-review.googlesource.com/c/go/+/418578
Run-TryBot: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go