tests: fix sigaction.test on sparc

tests: fix sigaction.test on sparc

On sparc, the first argument of old sigaction syscall has negated
semantics, compared to other architectures.  While sigaction.test
was aware about this arch-specific detail from the beginning, in one
of many sigaction syscall invocations a non-negative signal number
was erroneously passed on sparc.

* tests/sigaction.c (ADDR_INT, SIGNO_INT): New arch-specific macros.
(signo, addr): Use them.