[dev.power64] liblink: power64 fixes and ports of changes

Ports of platform-specific changes that happened on default
after dev.power64 forked (fixes for c2go, wrapper math fixes,
moved stackguard field, stackguard1 support, precise stacks).
Bug fixes (missing AMOVW in instruction table, correct
unsigned 32-bit remainder).

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/164920044