[dev.power64] cmd/ld: add MAXIO back in lib.h

It was accidentally removed.

LGTM=dave
R=rsc, iant, dave
CC=golang-codereviews
https://codereview.appspot.com/124880043
[dev.power64] runtime: rename traceback_arm.c to traceback_lr.c
Also make it more portable by not hardcoding pointer sizes.

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/127780043
[dev.power64] runtime: ignore rt_sigaction error if it's for SIGRTMAX.
Workaround a qemu linux user space emulation bug.
ref: http://git.qemu.org/?p=qemu.git;a=blob;f=linux-user/signal.c;h=1141054be2170128d6f7a340b41484b49a255936;hb=HEAD#l82

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/124900043
[dev.power64] syscall: remove duplicated constants

Without this CL, simply running mkall.sh for Linux will generate duplicated constants
for termios. I verified that after this CL, mkall.sh will generate almost identical
z* files for linux/amd64.

LGTM=rsc
R=rsc, iant, r
CC=golang-codereviews
https://codereview.appspot.com/124990043
[dev.power64] debug/elf: add power64 support along with relocation test

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/125910043
[dev.power64] cmd/cgo: add {ppc,power}64{,le} to pointer and int size maps

LGTM=rsc
R=rsc, iant, dave
CC=golang-codereviews
https://codereview.appspot.com/119640043
[dev.power64] cmd/9l: introduce the power64 linker, based loosely on 5l

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/123940043
[dev.power64] cmd/9l/9.out.h: introduce NFREG, REGG, REGRT1 and REGRT2

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/125990043
[dev.power64] cmd/9g: first check point

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/123170043