SSH GenerateKey very slow on ARM

Generating an SSH key is very slow on ARM due to unoptimised golang math functions. Several tests in utils/ssh that generate a key also end up extremely slow:

$ (cd utils/ssh/&&go test -gocheck.v -gocheck.f="(Generate|ClientKeys)Suite")
PASS: clientkeys_test.go:101: ClientKeysSuite.TestLoadClientKeysDirExists 26.505s
PASS: clientkeys_test.go:74: ClientKeysSuite.TestPrivateKeyFiles 64.608s
PASS: clientkeys_test.go:51: ClientKeysSuite.TestPublicKeyFiles 42.043s
PASS: generate_test.go:20: GenerateSuite.TestGenerate 24.356s
OK: 4 passed
PASS
ok launchpad.net/juju-core/utils/ssh 158.500s

$ head -n 6 /proc/cpuinfo
Processor : ARMv7 Processor rev 4 (v7l)
processor : 0
BogoMIPS : 1694.10

processor : 1
BogoMIPS : 1694.10

As the generation function reads from /dev/urandom this shouldn't be an entropy issue, benchmarking shows most time spent under ProbablyPrime in the math package. Lack of custom assembly is probably hurting here compared to amd64.

For the tests, the generation could probably be mocked out to not do all this work.