selftests/powerpc: Remove powerpc special cases from stack expansion test

Now that the powerpc code behaves the same as other architectures we
can drop the special cases we had.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724092528.1578671-5-mpe@ellerman.id.au

selftests/powerpc: Update the stack expansion test

Update the stack expansion load/store test to take into account the
new allowance of 4224 bytes below the stack pointer.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724092528.1578671-3-mpe@ellerman.id.au

selftests/powerpc: Add test of stack expansion logic

We have custom stack expansion checks that it turns out are extremely
badly tested and contain bugs, surprise. So add some tests that
exercise the code and capture the current boundary conditions.

The signal test currently fails on 64-bit kernels because the 2048
byte allowance for the signal frame is too small, we will fix that in
a subsequent patch.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724092528.1578671-1-mpe@ellerman.id.au