powerpc: don't use asm-generic/ptrace.h

Doing the indirection through macros for the regs accessors just
makes them harder to read, so implement the helpers directly.

Note that only the helpers actually used are implemented now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>