Rename locore.S to locore-v4.S and add a new locore-v6.S for starting up
armv6/7 systems.  We need to use some new armv6/7 features at startup
and splitting the implemenations to separate files will be more maintainable
than adding even more #ifdef sections to locore.S.

Because of the standardized interfaces to cache and MMU operations in armv7,
we can tolerate the kernel being entered with caches enabled.  This allows
running u-boot and loader(8) with caches enabled, and the performance
improvement can be dramatic (boot times can be cut from over a minute
to under 30 seconds).  The new implementation also has more robust cache
and mmu sequences for launching AP cores, and it paves the way for
upcoming changes to the pmap code which will use the TEX remap feature.

Changes in mp_machdep.c work with the new behavior in locore-v6 mp_entry,
and also reuse the original boot-time page tables to get transitioned
from physical to virtual addressing before installing the normal tables.

Submitted by Svatopluk Kraus and Michal Meloun with some changes by me.
Add a new locore.S that #includes the right (v4 or v6) implementation.
The kernel build machinery really wants the entry point to be in a file
named locore.S so doing this avoids a bunch of changes to the build system
for relatively little benefit.