[dev.boringcrypto] crypto/tls: restore AES-GCM priority when BoringCrypto is enabled

[dev.boringcrypto] crypto/tls: restore AES-GCM priority when BoringCrypto is enabled

This patch used to be in crypto/internal/cipherhw.AESGCMSupport which
was removed from the tree. It was meant and documented to affect only
crypto/tls, so move the logic there.

Change-Id: I36ed4f08a5fe2abaab18907910899ae0297d1611
Reviewed-on: https://go-review.googlesource.com/114816
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>