linux-user: Enable Signal Handlers on PPC64

linux-user: Enable Signal Handlers on PPC64

Enable the 64-bit PowerPC signal handling code that was previously
disabled via #ifdefs.  Specifically:

  - Move the target_mcontext (register save area) structure and
    append it to the 64-bit target_sigcontext structure.  This
    provides the space on the stack for saving and restoring
    context.
  - Define the target_rt_sigframe for 64-bit.
  - Adjust the setup_frame and setup_rt_frame routines to properly
    select the target_mcontext area and trampoline within the stack
    frame; tthis is different for 32-bit and 64-bit implementations.
  - Adjust the do_setcontext stub for 64-bit so that it compiles
    without warnings.

The 64-bit signal handling code is still not functional after this
change; but the 32-bit code is.  Subsequent changes will address
specific issues with the 64-bit code.

Signed-off-by: Tom Musta <tommusta@gmail.com>
[agraf: fix build on 32bit hosts, ppc64abi32]
Signed-off-by: Alexander Graf <agraf@suse.de>