[dev.power64] cmd/9c: fix switch bug

The cmd/cc bundled with 9c has additional code compared to our cmd/cc, and
without those code, 9c couldn't handle switch statement where the expression
is not just a simple int32 or int64 variable (e.g. g->status or int8 variable).

All credit goes to rsc.

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/125840043
[dev.power64] cmd/ld: update for power64

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/121380043
[dev.power64] liblink: handle $i64. symbols, add debug output for -S -v, show rel addend in hex for RISC arches

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/127770043
[dev.power64] cmd/9c: add Makefile

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/121370043