[dev.cc] 9l: make R_CALLPOWER like ELF's R_PPC64_REL24

[dev.cc] 9l: make R_CALLPOWER like ELF's R_PPC64_REL24

These accomplished the same thing, but R_CALLPOWER expected
the whole instruction to be in the addend (and completely
overwrote what was in the text section), while R_PPC64_REL24
overwrites only bits 6 through 24 of whatever was in the text
section.  Make R_CALLPOWER work like R_PPC64_REL24 to ease the
implementation of dynamic linking.

LGTM=rsc
R=rsc
CC=golang-codereviews, minux
https://golang.org/cl/177430043