pgsql: Fix using injection points at backend startup in EXEC_BACKEND mo

Fix using injection points at backend startup in EXEC_BACKEND mode

Commit 86db52a506 changed the locking of injection points to use only
atomic ops and spinlocks, to make it possible to define injection
points in processes that don't have a PGPROC entry (yet). However, it
didn't work in EXEC_BACKEND mode, because the pointer to shared memory
area was not initialized until the process "attaches" to all the
shared memory structs. To fix, pass the pointer to the child process
along with other global variables that need to be set up early.

Backpatch-through: 17

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b9e5249c29354186576d8fc00609fe7eaf7c8d25

Modified Files
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src/backend/postmaster/launch_backend.c | 12 ++++++++++++
src/backend/utils/misc/injection_point.c | 2 +-
src/include/utils/injection_point.h | 4 ++++
3 files changed, 17 insertions(+), 1 deletion(-)