Since Zephyr itself does not currently support processes, but
conformant applications should still be able to link, add stubs
for the remaining POSIX functions in the POSIX_SIGNALS Option
Group.
The POSIX_SIGNALS Option Group is required for PSE51, PSE52,
PSE53, PSE54, and likely many other POSIX Subprofiles.
Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>