linux-user: Split PPC Trampoline Encoding from Register Save

Split the encoding of the PowerPC sigreturn trampoline from the saving of
register state onto the signal handler stack.  This will make it easier
in subsequent patches to deal with variations in the stack frame layouts between
32 and 64 bit PowerPC.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>