powerpc/ptrace: Adapt gpr32_get, gpr32_set functions for transaction

This patch splits gpr32_get, gpr32_set functions to accommodate
in transaction ptrace requests implemented in patches later in
the series.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>