Recompute stack base in forked postmaster children.

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