pid1: make use of high rt signals on hppa with newer kernels

pid1: make use of high rt signals on hppa with newer kernels

Back in 4dffec1459f50ac9f8f67ccfcb79836b4ed5a50e we stopped using SIGRTMIN+26
and higher on hppa because they were not available. Then they became available
in linux 3.18:

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1f25df2eff5b25f52c139d3ff31bc883eee9a0ab

Instead of hard-coding the list based on architecture, let's use a runtime
check like signal(7) says.

(A note about implementation: RTSIG_IF_AVAILABLE is defined to take the full
signal and not just an offset from SIGRTMIN so that it's still possible to
grep for SIGRTMIN\+.)

Add a simple "test" to print the signal values.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=84931.