add sysconf keys/values for signal stack size

add sysconf keys/values for signal stack size

as a result of ISA extensions exploding register file sizes on some
archs, using a constant for minimum signal stack size no longer seems
viably future-proof. add sysconf keys allowing the kernel to provide a
machine-dependent minimum applications can query to ensure they
allocate sufficient space for stacks. the key names and indices align
with the same functionality in glibc.

see commit d5a5045382315e36588ca225889baa36ed0ed38f for previous
action on this subject.

ultimately, the macros MINSIGSTKSZ and SIGSTKSZ probably need to be
deprecated, but that is standards-amendment work outside the scope of
a single implementation.