fix: dev: Restore arc4random() detection dropped in the v9.21.14 merge

Commit 4db9e5d90e2 ("Use arc4random for CSPRNG when available", part
of the CVE-2025-40780 fix) guarded the arc4random() code paths in
lib/isc/random.h and lib/isc/random.c with HAVE_ARC4RANDOM and added
the corresponding function check to meson.build.  The manual conflict
resolution in merge c2a672bbaef ("Merge tag 'v9.21.14'") kept the code
changes but dropped the meson.build hunk, so HAVE_ARC4RANDOM was never
defined and platforms with arc4random() (macOS and the BSDs) silently
fell back to the internal ChaCha-based CSPRNG.  Restore the check.

Assisted-by: Claude:claude-fable-5
Merge branch 'mnowak/restore-arc4random-meson-check' into 'main'

See merge request isc-projects/bind9!12451