cmd/compile: make builds reproducible in presence of **byte and **int8
CL 39915 introduced sorting of signats by ShortString
for reproducible builds. But ShortString treats types
byte and uint8 identically; same for rune and uint32.
CL 39915 attempted to compensate for this by only
adding the underlying type (uint8) to signats in addsignat.
This only works for byte and uint8. For e.g. *byte and *uint,
both get added, and their sort order is random,
leading to non-reproducible builds.
One fix would be to add yet another type printing mode
that doesn't eliminate byte and rune, and use it
for sorting signats. But the formatting routines
are complicated enough as it is.
Instead, just sort first by ShortString and then by String.
We can't just use String, because ShortString makes distinctions
that String doesn't. ShortString is really preferred here;
String is serving only as a backstop for handling of bytes and runes.
The long series of types in the test helps increase the odds of
failure, allowing a smaller number of iterations in the test.
On my machine, a full test takes 700ms.