Make fixes and improvements

- Introduce __assert_disable global
- Improve strsignal() thread safety
- Make system call tracing thread safe
- Fix SO_RCVTIMEO / SO_SNDTIMEO on Windows
- Refactor DescribeFoo() functions into one place
- Fix fork() on Windows when TLS and MAP_STACK exist
- Round upwards in setsockopt(SO_RCVTIMEO) on Windows
- Disable futexes on OpenBSD which seem extremely broken
- Implement a better kludge for monotonic time on Windows

third_party/stb/stb_truetype.c

@@ -405,6 +405,7 @@ static wontreturn void stbtt__assert_fail(const char *x, int line) {
       longjmp(stbtt_jmpbuf, line);
    } else {
       __assert_fail(x, __FILE__, line);
+      abort();
    }
 }