powerpc/booke64: Align initial stack setting to match that of aim64's

Clang9/LLD9 appears to get quite confused with the instruction stream used
to obtain the tmpstack pointer, almost as though it thinks this is a C
function, so tries to optimize it. Since the AIM64 method doesn't use the
TOC to obtain the tmpstack, just follow that model, and lld won't get
confused.

Reported by: bdragon
MFC after: 2 weeks