posix: add stubs for signal.h functions that need process support

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>