Make EXEC_BACKEND more convenient on Linux and FreeBSD.

Make EXEC_BACKEND more convenient on Linux and FreeBSD.

Try to disable ASLR when building in EXEC_BACKEND mode, to avoid random
memory mapping failures while testing.  For developer use only, no
effect on regular builds.

This has been originally applied as of f3e7806 for v15~, but
recently-added buildfarm member gokiburi tests this configuration on
older branches as well, causing it to fail randomly as ASLR would be
enabled.

Suggested-by: Andres Freund <andres@anarazel.de>
Tested-by: Bossart, Nathan <bossartn@amazon.com>
Discussion: https://postgr.es/m/20210806032944.m4tz7j2w47mant26%40alap3.anarazel.de
Backpatch-through: 12