pgsql: Make EXEC_BACKEND more convenient on macOS.

Make EXEC_BACKEND more convenient on macOS.

It's hard to disable ASLR on current macOS releases, for testing with
-DEXEC_BACKEND. You could already set the environment variable
PG_SHMEM_ADDR to something not likely to collide with mappings created
earlier in process startup. Let's also provide a default value that
works on current releases and architectures, for developer convenience.

As noted in the pre-existing comment, this is a horrible hack, but
-DEXEC_BACKEND is only used by Unix-based PostgreSQL developers for
testing some otherwise Windows-only code paths, so it seems excusable.

Back-patch to all supported branches.

Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/20210806032944.m4tz7j2w47mant26%40alap3.anarazel.de

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a8096e30f4d08f2b90cc147b44b414191de6155f

Modified Files
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src/backend/port/sysv_shmem.c | 10 ++++++++++
1 file changed, 10 insertions(+)