selftests/powerpc: Don't save TOC by default in asm helpers

Thare are some asm helpers for creating/popping stack frames in
basic_asm.h. They always save/restore r2 (TOC pointer), but none of the
selftests change r2, so it's unnecessary to save it by default.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220627140239.2464900-5-mpe@ellerman.id.au