powerpc64: fix loader regression

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.

Reviewed by: tsoome
MFC after: 1 month
X-MFC-With: b4cb3fe0e39a
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D32027