chacha20poly1305: fix detection of BMI on amd64
chacha20poly1305: fix detection of BMI on amd64

This change detects BMI2 usability as an additional condition
to examine the usability of AVX2 version algorithm, fixes
the crash on the platfrom which supports AVX2 but not support BMI2.

Change-Id: I5438d4ec84265c79a51c1439265a33b1be04878a
Reviewed-on: https://go-review.googlesource.com/34852
Reviewed-by: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/crypto,golang/crypto,ScriptRock/crypto,golang/crypto,ScriptRock/crypto