test13/hello.M1 assumes r0 is 0 on process entry

[mwhudson]

I don't really understand the point of what the assembly is doing but it certainly crashes if r0 is not 0. And the "64-Bit ELF V2 ABI Specification" at https://files.openpower.foundation/s/cfA2oFPXbbZwEBK in section 4.1.2.1 says "The contents of most registers are not specified when a process is first entered from an exec system call. A program should not expect the operating system to set all registers to 0." and provides no specfication for r0.

The build fails on ubuntu currently https://launchpad.net/ubuntu/+source/mescc-tools/1.5.2-1/+build/28152192 but used to succeed. It still succeeds on Debian but I have no idea why there is a difference, but given the above I'm not super interested in finding out.