selftests/powerpc: Rework FPU stack placement macros and move to header file

The FPU regs are placed at the top of the stack frame. Currently the
position expected to be passed to the macro. The macros now should be
passed the stack frame size and from there they can calculate where to
put the regs, this makes the use simpler.

Also move them to a header file to be used in an different area of the
powerpc selftests

Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>