hash/crc32: fix nil Castagnoli table problem

hash/crc32: fix nil Castagnoli table problem

When SSE is available, we don't need the Table. However, it is
returned as a handle by MakeTable. Fix this to always generate
the table.

Further cleanup is discussed in #16909.

Change-Id: Ic05400d68c6b5d25073ebd962000451746137afc
Reviewed-on: https://go-review.googlesource.com/27934
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>