scripts: Add ppc64le support for checkstack.pl

scripts: Add ppc64le support for checkstack.pl

64-bit ELF v2 ABI specification for POWER describes, on section "General
Stack Frame Requirements", that the stack should use the following
instructions when compiled with backchain:

  mflr r0
  std  r0, 16(r1)
  stdu r1, -XX(r1)

Where XX is the frame size for that function, and this is the value
checkstack.pl will find the stack size for each function.

This patch also simplifies the entire Powerpc section, since just two
type of instructions are used, 'stdu' for 64 bits and 'stwu' for 32 bits
platform.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>