chacha20poly1305: fix build constraints

Similarly to https://go-review.googlesource.com/32311,
chacha20poly1305 needs additional build constraints
for non-standard toolchains.

Change-Id: I22816ef333c05450e9ab4debb3ce591518b6b84b
Reviewed-on: https://go-review.googlesource.com/32391
Run-TryBot: Alex Vaghin <ddos@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
3 files changed