cmd/compile: only sort methods/interfaces during export for -d=unifiedquirks

These sorts are only important for 'toolstash -cmp' testing of unified
IR against -G=0 mode, but they were added before I added
-d=unifiedquirks to allow altering small "don't care" output details
like this.

This CL should help mitigate issues with #44195 until package
objectpath is updated and deployed.

Change-Id: Ia3dcf359481ff7abad5ddfca8e673fd2bb30ae01
Reviewed-on: https://go-review.googlesource.com/c/go/+/343390


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>