openrisc: Add HAVE_REGS_AND_STACK_ACCESS_API support

Support for HAVE_REGS_AND_STACK_ACCESS_API needed for restartable
sequences.

The implementation has been copied from riscv and tested with the
restartable sequences self tests.

Note, pt-regs members are 'long' on openrisc which require casts for the
api, someday we should try to update these to be 'unsigned long' as
that's what they really are.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
[stafford: Updated commit message]
Signed-off-by: Stafford Horne <shorne@gmail.com>