cmd/compile: use stringer on types.EType

cmd/compile: use stringer on types.EType

Another one that is possible thanks to the new -trimprefix stringer
flag.

The only subtle difference is that, in the previous version, some values
such as TUNSAFEPTR were stringified as "TUNSAFEPTR" instead of
"UNSAFEPTR". The new String method is always consistent in removing the
"T" prefix.

Passes toolstash -cmp on std cmd.

Change-Id: I68407f391795403dfcbbfa68c813018c0235bbb5
Reviewed-on: https://go-review.googlesource.com/77250
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>