A syscall has been reserved for a debug setcontext for PPC, but it has 
not yet been added.  This patch adds that function.  This syscall allows 
signal handlers to perform debug functions.  It allows the signal 
handler to turn on single-stepping, for instance, and the thread will 
get a trap after executing the next instruction.  It can also (on 
supported PPC processors) turn on branch tracing and get a trap after 
the next branch instruction is executed.  This is useful for 
in-application debugging.

A patch and a small demonstration program is attached.

This has been posted on linux-ppc and it seems to be ok with them.  I 
have tested gdb with this patch and it was not broken (any more than 
before), both on classic and Book E PowerPC.

-Corey