powerpc Linux scv support and scv system call ABI proposal

[Florian Weimer]

* Segher Boessenkool:

> On Wed, Jan 29, 2020 at 05:19:19PM +0100, Florian Weimer wrote:
>> * Segher Boessenkool:
>> >> But GCC doesn't expose them as integers to C code, so you can't do much
>> >> without them.
>> >
>> > Sure, it doesn't expose any other registers directly, either.
>> 
>> I can use r0 & 1 with a register variable r0 to check a bit.
>
> That is not reliable, or supported, and it *will* break.  This is
> explicit for local register asm, and global register asm is
> underdefined.

Ugh.  I did not know that.  And neither did the person who wrote
powerpc64/sysdep.h because it uses register variables in regular C
expressions. 8-(  Other architectures are affected as well.

One set of issues is less of a problem if system call arguments are
variables and not complex expressions, so that side effects do not
clobber registers in the initialization:

	register long __a0 asm ("$4") = (long) (arg1);

But I wasn't aware of that constraint on the macro users at all.

Thanks,
Florian