crypto/internal: keep fips140/aes.NewCTR from allocating

crypto/internal: keep fips140/aes.NewCTR from allocating

Return a *CTR from an always-inlineable function, so the allocation
can be lifted to the callsite.

Put the potentially uninlineable code in a separate function that returns a CTR.

Fixes #70499

Change-Id: I2531a2516e4c00aba65407f3bc24a7c21dd8f842
Reviewed-on: https://go-review.googlesource.com/c/go/+/631317
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>