cmd/internal/objabi: fix the bug of shrinking SymType down to a uint8

cmd/internal/objabi: fix the bug of shrinking SymType down to a uint8

Previous CL (cmd/internal/objabi: shrink SymType down to a uint8) shrinks
SymType down to a uint8 but forgot making according change in goobj.

Fixes #20296
Also add a test to catch such Goobj format inconsistency bug

Change-Id: Ib43dd7122cfcacf611a643814e95f8c5a924941f
Reviewed-on: https://go-review.googlesource.com/42971
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>