cmd/internal/objabi: declare HeadType String on a non-pointer receiver

cmd/internal/objabi: declare HeadType String on a non-pointer receiver

objabi.HeadType is typically used as a non-pointer type, however the String function
is declared on a pointer receiver. This means that in most cases its integer value
is printed, rather than the value from the String function.

Change-Id: I3d28d9680e88a714bc1152ed5e1df4ac43d7a33f
Reviewed-on: https://go-review.googlesource.com/c/go/+/430556
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Jenny Rakoczy <jenny@golang.org>