powerpc64: fix loader regression

[luporl]

Summary

After b4cb3fe0e39a, loader started crashing on PowerPC64, with a
Program Exception (700) error. The problem was that archsw was
used before being initialized, with the new mount feature. This
change fixes the issue by initializing archsw earlier, before
setting currdev, that triggers the mount.

Test Plan

Tested on POWER9 VM, booting successfully with both Hash and Radix MMUs.

[tsoome]

I think, it is still not set early enough; you want to have it set up before we run devsw probes.

[luporl]

Initialize archsw earlier.