cmd/compile: remove FS debug hash form

cmd/compile: remove FS debug hash form

The FS form was only necessary for reliable hashes in tests,
and for that we can use -trimpath.

Another potential concern would be temporary work directory
names leaking into the names of files generated by cgo and the
like, but we already make sure to avoid those to ensure
reproducible builds: the compiler never sees those paths.
So the FS form is not necessary for that either.

Change-Id: Idae2c6acb22ab64dfb33bb053244d23fbe153830
Reviewed-on: https://go-review.googlesource.com/c/go/+/493737
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>