We can't use `getCanonArch()` after all.

Under a Fedora 18 machine for which the `cpuarch` grain reports `x86_64`:

```python
>>> import rpmUtils.arch
>>> rpmUtils.arch.getCanonArch()
'ia32e'
>>> rpmUtils.arch.legitMultiArchesInSameLib()
['x86_64', 'amd64', 'ia32e']
>>> rpmUtils.arch.getBaseArch()
'x86_64'
>>>
```

Under a CentOS 6.4 for which the `cpuarch` grain reports `i686`:
```
>>> import rpmUtils.arch
>>> rpmUtils.arch.getCanonArch()
'i686'
>>> rpmUtils.arch.legitMultiArchesInSameLib()
['i686']
>>> rpmUtils.arch.getBaseArch()
'i386'
>>>
```

The only common recipe seems to be using `legitMultiArchesInSameLib()`