[musl] Powerpc Linux 'scv' system call ABI proposal take 2

[Florian Weimer]

* Rich Felker:

> My preference would be that it work just like the i386 AT_SYSINFO
> where you just replace "int $128" with "call *%%gs:16" and the kernel
> provides a stub in the vdso that performs either scv or the old
> mechanism with the same calling convention.

The i386 mechanism has received some criticism because it provides an
effective means to redirect execution flow to anyone who can write to
the TCB.  I am not sure if it makes sense to copy it.