Cannot build trusty armhf with go1.2 on from master

[Curtis Hovey]

juju-core failed to build on armhf for precise, trusty, and vivid. Deej in webops capture the failure for the precise armhf build for us https://pastebin.canonical.com/143418/
There is an error starting in line 1226 :

    src/golang.org/x/crypto/poly1305/poly1305_arm.s:26 syntax error, last name: R9

We know 1.25.0 built using:

    golang.org/x/crypto git c57d4a71915a248dbad846d60825145062b4c18e 2015-03-27T05:11:19Z

This 1.26-alpha1 package uses:

    golang.org/x/crypto git aedad9a179ec1ea11b7064c57cbc6dc30d7724ec 2015-08-30T18:06:42Z

[Cheryl Jennings (cherylj)]

https://github.com/juju/juju/pull/3673 reverted the crypto dependency back to c57d4a71915a248dbad846d60825145062b4c18e