cmd/compile, etc: handle many struct fields

This adds 8 bytes of binary size to every type that has methods. It is
the smallest change I could come up with for 1.7.

Fixes #16037

Change-Id: Ibe15c3165854a21768596967757864b880dbfeed
Reviewed-on: https://go-review.googlesource.com/24070
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/type.go

@@ -323,7 +323,9 @@ type method struct {
 type uncommontype struct {
 	pkgpath nameOff
 	mcount  uint16 // number of methods
-	moff    uint16 // offset from this uncommontype to [mcount]method
+	_       uint16 // unused
+	moff    uint32 // offset from this uncommontype to [mcount]method
+	_       uint32 // unused
 }
 
 type imethod struct {