Recent FreeBSD versions use LLVM's lld as a linker, not the GNU ld.
Their behavior slightly differs, so adapt the build system to handle
either one.
* The LLVM's linker prints "undefined symbol:" instead of
"undefined reference to". Tweak tools/mockup.sh to also look for that
message.
* The LLVM's linker may only print the first dozen errors (omitting
the rest to avoid flooding the screen). tools/update-mocks.sh relies
on getting all errors as it extracts the missing symbols' names from
the error output and creates mocks for them. Detect if errors were
omitted and re-run, telling the linker to not omit any. The GNU linker
does not support -error-limit=0, so unfortunately we can't just run
with that option unconditionally from the first attempt.
* Nit: FreeBSD's sed(1) prints "t" for "\t" instead of a horizontal tab.
Use a verbatim tab in the command, instead of "\t" which works on
both.
Changelog-Fixed: Developer mode compilation on FreeBSD.
b2c4d5e952