Add support for building Book-E kernels with clang/lld.

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