New powerpc vdso calling convention

[Nicholas Piggin]

As noted in the 'scv' thread, powerpc's vdso calling convention does not 
match the C ELF ABI calling convention (or the proposed scv convention).
I think we could implement a new ABI by basically duplicating function
entry points with different names.

The ELF v2 ABI convention would suit it well, because the caller already
requires the function address for ctr, so having it in r12 will 
eliminate the need for address calculation, which suits the vdso data 
page access.

Is there a need for ELF v1 specific calls as well, or could those just be 
deprecated and remain on existing functions or required to use the ELF 
v2 calls using asm wrappers?

Is there a good reason for the system call fallback to go in the vdso 
function rather than have the caller handle it?

Thanks,
Nick