[9.20] chg: nil: Suppress FreeBSD-specific TSAN false-positive data race

TSAN reports a data race in FreeBSD's memset(), called by its
__crt_calloc() memory allocation function. There is a very similar
bug report [1] in FreeBSD bug tracker, and an existing code-review [2]
that tries to address an issue, the description of which is very
similar to what we are seeing.

Suppress this report by adding its signature to '.tsan-suppress'.

[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=282794

[2] https://reviews.freebsd.org/D28536?id=86694

Closes #5267

Backport of MR !10353

Merge branch 'backport-5267-freebsd-suppress-tsan-data-race-false-positive-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10354

.tsan-suppress

@@ -1,3 +1,6 @@
 # Uninstrumented libraries
 called_from_lib:libfstrm.so
 race:dummyrpz
+
+# FreeBSD false-positive, see GL#5267
+race:libexec/rtld-elf/rtld_malloc.c