TSAN doesn't support real-time signals on FreeBSD

[tavianator]

tavianator@muon $ cat foo.c
#include <signal.h>
#include <stdio.h>

int main(void) {
        if (signal(SIGRTMIN, SIG_DFL) == SIG_ERR) {
                perror("signal()");
        }
        return 0;
}
tavianator@muon $ clang18 foo.c -o foo
tavianator@muon $ ./foo
tavianator@muon $ clang18 -fsanitize=thread foo.c -o foo
tavianator@muon $ ./foo
signal(): Invalid argument
tavianator@muon $ clang18 --version
clang version 18.1.3
Target: x86_64-portbld-freebsd14.0
Thread model: posix
InstalledDir: /usr/local/llvm18/bin
tavianator@muon $ uname -a
FreeBSD muon 14.0-RELEASE-p5 FreeBSD 14.0-RELEASE-p5 #0: Tue Feb 13 23:37:36 UTC 2024     root@amd64-builder.daemonology.net:/usr/obj/usr/src/amd64.amd64/sys/GENERIC amd64

I think kSigCount is wrong here:

llvm-project/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp

Line 64 in ee765b0

const int kSigCount = 65;