xtensa: fix layout of struct sigcontext to match the kernel

Current code breaks libgcc unwind.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Chris Zankel <chris@zankel.net>
xtensa: fix atomic_decrement_if_positive() return value

atomic_decrement_if_positive() returns the old value of &mem, not the
(sometimes undefined) value of __tmp.

Fixes the uClibc nptl/tst-sem3 test.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Chris Zankel <chris@zankel.net>