pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/fc3f4453a2bc95549682e23600b22e658cb2d6d7

Modified Files
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src/backend/postmaster/postmaster.c | 5 -----
src/backend/utils/init/miscinit.c | 9 +++++++++
2 files changed, 9 insertions(+), 5 deletions(-)