This involved several changes:
* Since lld does not like text relocations, replace SMP boot page text relocs
in booke/locore.S with position-independent math, and track the virtual base
in the SMP boot page header.
* As some SPRs are interpreted differently on clang due to the way it handles
platform-specific SPRs, switch m*dear and m*esr mnemonics out for regular
m*spr. Add both forms of SPR_DEAR to spr.h so the correct encoding is selected.
* Change some hardcoded 32 bit things in the boot page to be pointer-sized, and
fix alignment.
* Fix 64-bit build of booke/pmap.c when enabling pmap debugging.
Additionally, I took the opportunity to document how the SMP boot page works.
Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D21999
ab3f2a3861