devel/libunwind: set LLD_UNSAFE

libunwind.so has symbols with ELF protected visibility which various
test applications could preempt. Both GNU gold and LLVM's lld fail to
link this case, with errors like

lld:
/usr/bin/ld: error: cannot preempt symbol: _ULx86_64_init_local

defined in ../src/.libs/libunwind.so
referenced by Lperf-trace.c

Lperf-trace.o:(main)

gold:
/usr/local/bin/ld.gold: error: Gperf-simple.o: cannot make copy relocation

for protected symbol '_Ux86_64_local_addr_space', defined in
../src/.libs/libunwind-x86_64.so

For now prevent libunwind from linking with lld. More information can be
found in libunwind upstream issue 58
https://github.com/libunwind/libunwind/issues/58

PR: 219524
Approved by: portmgr (LLD_UNSAFE blanket)
Sponsored by: The FreeBSD Foundation