fix build for Linux/ARM.

fix build for Linux/ARM.
        1, IMO, the fatal error "regfree: not a register" from 5g when
         compiling runtime/debug.go is due to gcc miscompile, it doesn't
         show up when compiled with -O0. But I still haven't thought of
         a way to fix this, should all ARM builds be built with -O0?
        2, fixed mksysnum_linux.pl, so zsysnum_linux_arm.go no longer
        needs to be hand-generated.
        3, regen all in pkg syscall for Linux/ARM on Debian 6.0
This CL is somewhat big, I'd like to split it if necessary.

R=rsc, dave
CC=golang-dev
https://golang.org/cl/5659044