Add missing symbols to the uksignal interface

Add missing symbols to the uksignal interface

The Go runtime requires some symbols related to signals that are not
currently defined in `signal.h` (after the uksignal support was added).
Changes brought in this commit:
- add missing fields to `siginfo_t`
- declare `sigaltstack` (the stub implementation is added to uksignal
in a separate pull request, part of the core Unikraft repository)
- add symbols for `ss_flags` values

Signed-off-by: Răzvan Vîrtan <virtanrazvan@gmail.com>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Pull-Request: #11