powerpc/exec: Set thread.regs early during exec

In later patches during exec, we would like to access default regs.amr to
control access to the user mapping. Having thread.regs set early makes the
code changes simpler.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201127044424.40686-10-aneesh.kumar@linux.ibm.com