libgcc: Add a backchain fallback to _Unwind_Backtrace() on PowerPC

libgcc: Add a backchain fallback to _Unwind_Backtrace() on PowerPC

Without dwarf2 unwind tables available _Unwind_Backtrace() is not
able to return the full backtrace.
This patch adds a fallback function on powerpc to get the backtrace
by doing a backchain, this code was originally at glibc.

libgcc/ChangeLog:

	* config/rs6000/linux-unwind.h (struct rt_sigframe): Move it to
	outside of get_regs() in order to use it in another function, this
	is done twice: for __powerpc64__ and for !__powerpc64__.
	(struct trace_arg): New struct.
	(struct layout): New struct.
	(ppc_backchain_fallback): New function.
	* unwind.inc (_Unwind_Backtrace): Look for _URC_NORMAL_STOP code
	state and call MD_BACKCHAIN_FALLBACK.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/unwind-backchain.c: New test.