cmd/compile: use ReadFull to read fingerprint

Don't fail on partial read.

May fix #38849.

Change-Id: Icf075d454e1bfe9299b07eea47bbc4d448c3bd5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/232317


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>