lib/uklibid: Supporting pre-built libraries

lib/uklibid: Supporting pre-built libraries

This commit introduces a clearer separation between runtime and
compile-time resolution of library identifiers with the API. This is done
so that libraries can potentially be built off-tree and shipped in binary
form. Each library's own identifier (`uk_libid_self()`) is stored in a
constant variable that is resolved during link-time. The owner of this
variable is `uklibid`, so the identifier is external to a binary library.

A binary delivery of a library can be created with Unikraft (for example:
`make libbinary`) and the resulting library object (likely under
`build/libbinary.o`) can be delivered with a `Config.uk` like:
```KConfig
config LIBBINARY
	bool "Binary example library"
	default n
```

and a `Makefile.uk` like:
```Makefile
UK_OLIBS-$(CONFIG_LIBBINARY) += $(IMPORT_BASE)/libbinary.o
UK_LIBID_EXTRA-$(CONFIG_LIBBINARY) += libbinary
```

Please note that the library must be registered directly in `UK_OLIBS-y`;
`addlib` or `addlib_s` cannot be used in such cases.
Please also note that such a library should restrict dependencies on other
libraries and their configuration options. In particular, the use of macros
and inline functions must be handled with care.

Signed-off-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Robert Kuban <robert.kuban@opensynergy.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #938