tests: add a test for rt_sigaction output

Since "struct sigaction" varies between architectures, rt_sigaction
decoding sometimes produces incorrect output.  This test is expected
to catch basic rt_sigaction decoding bugs.

Based on a patch proposed by Chris Dearman.

* tests/sigaction.c: New file.
* tests/sigaction.awk: Likewise.
* tests/sigaction.sh: New test.
* tests/Makefile.am (check_PROGRAMS): Add sigaction.
(TESTS): Add sigaction.sh.
(EXTRA_DIST): Add sigaction.awk.
* tests/.gitignore: Add sigaction.
Factor out struct sigaction printing code

* signal.c [HAVE_SIGACTION] (decode_old_sigaction): New function.
[HAVE_SIGACTION] (sys_sigaction): Use it.
(decode_new_sigaction): New function.
(sys_rt_sigaction): Use it.
mips: fix sigaction reporting

MIPS userland uses the same sigaction structure with a full signal
mask for old_sigaction and new_sigaction and does does not have
an sa_restorer field.

These changes have been tested on MIPS O32 big/little endian, MIPS N64
big endian and x86-64.

* signal.c (old_sigaction) [MIPS]: Add definition for MIPS.
(decode_old_sigaction) [MIPS]: Print sa_mask according to its definition.
(new_sigaction) [MIPS]: Add definition for MIPS.

Signed-off-by: Chris Dearman <chris.dearman@imgtec.com>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>