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/4853
Reviewed-by: Minux Ma <minux@golang.org>

diff --git a/src/runtime/gcwork.go b/src/runtime/mgcwork.go
similarity index 100%
rename from src/runtime/gcwork.go
rename to src/runtime/mgcwork.go