crypto/internal/fips/aes/gcm: fix PPC64 gcm debug option logic

crypto/internal/fips/aes/gcm: fix PPC64 gcm debug option logic

AES-GCM is always supported, unless the debug option explicitly
turns it off.

Change-Id: Iac17fd8429307d48daa601249a2505bcd1b1f892
Reviewed-on: https://go-review.googlesource.com/c/go/+/629695
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Archana Ravindar <aravinda@redhat.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>