runtime: rename gcwork.go to mgcwork.go

All of the other memory-related source files start with "m".  Keep up
the tradition.

Change-Id: Idd88fdbf2a1453374fa12109b949b1c4d149a4f8
Reviewed-on: https://go-review.googlesource.com/4853Reviewed-by: Minux Ma <minux@golang.org>