cmd/compile: avoid static init of strings in FIPS mode

cmd/compile: avoid static init of strings in FIPS mode

Strings have relocations, and data relocations are bad.
Other literals are fine.

Fixes build failure in pending CL 628776.

Change-Id: I7a38bbff9776a365c5823d54c4a00e068dda5d9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/628915
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Russ Cox <rsc@golang.org>