cmd/objdump: reference tracking bug in TestDisasmCode skip message

[sthagen]

Issue golang#12559 was closed and split into golang#19158 for mips{,le} and golang#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

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