gcc: backport xtensa libgcc stack unwinding fixes

These backported patches fix the following failing uClibc-ng tests:
  nptl/tst-cancelx4,
  nptl/tst-cancelx16,
  nptl/tst-cancelx18,
  nptl/tst-cancelx20,
  nptl/tst-cancelx21,
  nptl/tst-cleanupx1,
  nptl/tst-cleanupx3,
  nptl/tst-oncex3,
  nptl/tst-oncex4.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>