[libc++] FreeBSD CI: Adds `<signal.h>` to `check_assertion.h` (#78863)

...in attempt to fix the FreeBSD CI.

I noticed that suddenly some tests in the latest PRs fail to compile on
FreeBSD (`SIGILL` and `SIGTRAP` not defined). This tries to resolve
the issue.

Co-authored-by: Zingam <zingam@outlook.com>