Use SIGRTMIN from kernel headers

* configure.ac (ASM_SIGRTMIN): Define to SIGRTMIN from <asm/signal.h>.
* signal.c: Use ASM_SIGRTMIN instead of constants provided by libc.
* tests/sigreturn.c: Use ASM_SIGRTMIN instead of hardcoded value.
Use lower RT_* numbers to support pre-3.18 hppa kernels.
* tests/sigreturn.test: Update regexp.
4 files changed