cmd/objdump: reference tracking bug in TestDisasmCode skip message

Issue #12559 was closed and split into #19158 for mips{,le} and #19156
for mips64{,le}. Instead of referencing the individual GOARCH-specific
issues in the skip test messages of TestDisasmCode use the tracking bug

Change-Id: I6929d25f4ec5aef4f069b7692c4e29106088ce65
Reviewed-on: https://go-review.googlesource.com/c/go/+/209817


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>