linux-user: ppc signal handling

Implement setup_{,rt_}frame and do_{,rt_}sigreturn for PPC 32-bit.  Use
the same TARGET_QEMU_ESIGRETURN hack as for MIPS to avoid clobbering
register state on a sigreturn.

Signed-off-by: Nathan Froyd <froydnj@codesourcery.com>
Signed-off-by: malc <av1474@comtv.ru>
