selftests/powerpc/ptrace: Add peek/poke of FPRs

Currently the ptrace-gpr test only tests the GET/SET(FP)REGS ptrace
APIs. But there's an alternate (older) API, called PEEK/POKEUSR.

Add some minimal testing of PEEK/POKEUSR of the FPRs. This is sufficient
to detect the bug that was fixed recently in the 32-bit ptrace FPR
handling.

Depends-on: 8e127844 ("powerpc/32: Fix overread/overwrite of thread_struct via ptrace")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220627140239.2464900-13-mpe@ellerman.id.au