libdwarf: Add a weak uncompress() symbol

This works around brokenness in buildworld's bootstrapping logic: it
uses the source tree's metadata to collect dependency info (such as,
"libdwarf depends on libz") but links against static host libraries.
If these two are out of sync, as is the case if one builds a commit
prior to the introduction of the libz dependency, then the build fails
when trying to statically link nm(1).

Mitigate the problem by defining a weak uncompress() symbol which simply
returns an error. This ensures that the build won't fail when
statically linking libdwarf without zlib. The downside is that any
tools using libdwarf without zlib will now hit a runtime error if they
attempt to decode compressed sections, but at least they'll fail
deterministically, and compressed debug info is only enabled by default
in main.

In particular, this fixes building of branches lacking commit
dbf05458e3bd, such as releng branches, stable/12 and 13 and old
revisions of main. Previously the nm(1) build would fail with:

ld: error: undefined symbol: uncompress
>>> referenced by libdwarf_elf_init.c:233
>>> (/usr/src/contrib/elftoolchain/libdwarf/libdwarf_elf_init.c:233)
>>> libdwarf_elf_init.o:(_dwarf_elf_init) in archive
>>> /usr/lib/libdwarf.a

Reported by: dim, ler, krion
Reviewed by: imp, emaste
Fixes: dbf05458e3bd ("libdwarf: Support consumption of compressed ELF sections")
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33419