Recompute stack base in forked postmaster children.
This is for the benefit of running postgres under the rr
debugger. When using rr signal handlers running while a syscall is
active use an alternative stack. As e.g. bgworkers are started from
within signal handlers, the forked backend then has a different stack
base than postmaster. Previously that subsequently lead to those
processes triggering spurious "stack depth limit exceeded" errors.
Discussion: https://postgr.es/m/
20200327182217.ubrrl32lyfhxfwk5@alap3.anarazel.de