Flesh out the SIG* constants

This is a follow-up to #5761. Its purpose is to make core::libc more consistent - it currently only defines SIGKILL and SIGTERM, because they are the only values that happen to be needed by libcore.

This adds all the posix signal value constants, except for those that have different values on different architectures.

The output of the command `man 7 signal` was used to compile these signal values.