libc: gen: add a test for rtld underflowing our posix_spawn thread

This is a distillation of the environment described in the PR, using
a dummy shlib and mapping it repeatedly. This takes advantage of the
guard page added in 2767a1f3686e5b16 to reliably crash if rtld tries to
scale its stack usage excessively with the # DSOs loaded.

PR: 295991
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D57954