use kernel-provided AT_MINSIGSTKSZ for sysconf(_SC_[MIN]SIGSTKSZ)

use kernel-provided AT_MINSIGSTKSZ for sysconf(_SC_[MIN]SIGSTKSZ)

use the legacy constant values if the kernel does not provide
AT_MINSIGSTKSZ (__getauxval will return 0 in this case) and as a
safety check if something is wrong and the provided value is less than
the legacy constant.

sysconf(_SC_SIGSTKSZ) returns SIGSTKSZ adjusted for the difference
between the legacy constant MINSIGSTKSZ and the runtime value, so that
the working space the application has on top of the minimum remains
invariant under changes to the minimum.