This is a more robust method for obtaining the availability of vx.
Since this variable may be checked frequently I've also now
padded it so that it will be in its own cache line.

I've kept the other check (in hash/crc32) the same for now until
I can figure out the best way to update it.

Updates #15403.

Change-Id: I74eed651afc6f6a9c5fa3b88fa6a2b0c9ecf5875
Reviewed-on: https://go-review.googlesource.com/31149Reviewed-by: Austin Clements <austin@google.com>