Use blivet.arch.getArch() to get current arch

The targetarch flag & accompanying boot option have been recently dropped by Anaconda as the flag was no longer being used by Anaconda code and the boot option was no longer supplied by the tools that use Anaconda.

Also unlike targetarch that depended on the the target architecture being set on boot command line, blivet.arch.getArch() returns the actual architecture of the current environment. So getArch() will actually work in a ppc64 environment without any special boot options, unlike targetarch.