runtime: reorganize memory code

Move code from malloc1.go, malloc2.go, mem.go, mgc0.go into
appropriate locations.

Factor mgc.go into mgc.go, mgcmark.go, mgcsweep.go, mstats.go.

A lot of this code was in certain files because the right place was in
a C file but it was written in Go, or vice versa. This is one step toward
making things actually well-organized again.

Change-Id: I6741deb88a7cfb1c17ffe0bcca3989e10207968f
Reviewed-on: https://go-review.googlesource.com/5300
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/heapdump.go

@@ -13,6 +13,24 @@ package runtime
 
 import "unsafe"
 
+//go:linkname runtime_debug_WriteHeapDump runtime/debug.WriteHeapDump
+func runtime_debug_WriteHeapDump(fd uintptr) {
+	semacquire(&worldsema, false)
+	gp := getg()
+	gp.m.preemptoff = "write heap dump"
+	systemstack(stoptheworld)
+
+	systemstack(func() {
+		writeheapdump_m(fd)
+	})
+
+	gp.m.preemptoff = ""
+	gp.m.locks++
+	semrelease(&worldsema)
+	systemstack(starttheworld)
+	gp.m.locks--
+}
+
 const (
 	fieldKindEol       = 0
 	fieldKindPtr       = 1