[dev.power64] reflect: fix asm on power64x

reflect/asm_power64x.s was missing changes made to other
platforms for stack maps.  This CL ports those changes.  With
this fix, the reflect test passes on power64x.

LGTM=rsc
R=rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/170870043