build: Make the directory for helper executables consistent with Autotools

The Autotools build system has been using /usr/lib/polkit-1 for several
releases, even on distributions where the library directory is /usr/lib64
or /usr/lib/x86_64-linux-gnu, so it makes sense for Meson to do the same.
This lets 32- and 64-bit polkit agents share a single helper executable.

This might be superseded by polkit!63, which requests going back to using
the libexecdir for these (like polkit 0.105 did), which would also make
sense; but until that's decided, let's at least be consistent between
our two build systems.

Every time we change this, all programs that have already loaded
libpolkit-agent into their address space need to be restarted, unless
distributions provide compatibility symlinks.

Signed-off-by: Simon McVittie <smcv@debian.org>
2 jobs for meson-libprivdir in 5 minutes and 46 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #10769504
build_autotools

00:05:45

passed #10769503
build_stable

00:03:28