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.

1 files changed, 24 insertions, 0 deletions

diff --git a/gcc/testsuite/gcc.target/powerpc/unwind-backchain.c b/gcc/testsuite/gcc.target/powerpc/unwind-backchain.c
new file mode 100644
index 0000000..affa9b2
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/unwind-backchain.c
@@ -0,0 +1,24 @@
+/* -linux* targets have a fallback for the absence of unwind tables, thus are
+   the only ones we can guarantee backtrace returns all addresses.  */
+/* { dg-do run { target { *-*-linux* } } } */
+/* { dg-options "-fno-asynchronous-unwind-tables" } */
+
+#include <execinfo.h>
+
+void
+test_backtrace()
+{
+  int addresses;
+  void *buffer[10];
+
+  addresses = backtrace(buffer, 10);
+  if(addresses != 4)
+    __builtin_abort();
+}
+
+int
+main()
+{
+  test_backtrace();
+  return 0;
+}