libdwarf: Add a weak uncompress() symbol

[markj]

Summary

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.

Reported by: dim, ler, krion
Fixes: dbf05458e3bd ("libdwarf: Support consumption of compressed ELF sections")
Sponsored by: The FreeBSD Foundation

Test Plan

stable/12 builds again if I manually build and install the patched libdwarf on my system.

I verified that libdwarf consumers can still load compressed data when linked against
libdwarf.so. Consumers linked with libdwarf.a and not libz.a get an error when initializing
dwarf info, as expected.

[emaste]

I think this is a fine workaround, but we really ought to remove after either 12.3 and 13.0 are past EOL, or once we address the bootstrap issue somehow.

contrib/elftoolchain/libdwarf/zlib_stub.c

16

Perhaps mention the specific older releases here?

[imp]

Echo Ed's comments...

contrib/elftoolchain/libdwarf/zlib_stub.c

22

Don't we have a cdefs.h define for this?

[markj]

Note affected release versions, use __weak_symbol from cdefs.h,
drop an unneeded include.

[markj]