powerpc: remove STACK_FRAME_OVERHEAD

This is equal to STACK_FRAME_MIN_SIZE on 32-bit and 64-bit ELFv1, and no
longer used in 64-bit ELFv2, so replace STACK_FRAME_OVERHEAD occurrences
with STACK_FRAME_MIN_SIZE.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221127124942.1665522-18-npiggin@gmail.com