runtime: convert mprof.goc to mprof.go

The exported Go definitions appearing in mprof.go are
copied verbatim from debug.go.

The unexported Go funcs and types are new.
The C Bucket type used a union and was not a line-for-line translation.

LGTM=remyoudompheng
R=golang-codereviews, remyoudompheng
CC=dvyukov, golang-codereviews, iant, khr, r
https://golang.org/cl/137040043
diff --git a/src/pkg/runtime/malloc.go b/src/pkg/runtime/malloc.go
index 49afc67..5b1825a 100644
--- a/src/pkg/runtime/malloc.go
+++ b/src/pkg/runtime/malloc.go
@@ -397,9 +397,8 @@
 		}
 		c.next_sample = next
 	}
-	mp.scalararg[0] = uintptr(size)
-	mp.ptrarg[0] = x
-	onM(&mprofMalloc_m)
+
+	mProf_Malloc(x, size)
 }
 
 // force = 1 - do GC regardless of current heap usage