Suppress FreeBSD-specific TSAN false-positive data race

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