error: undefined symbol: _Unwind_Backtrace on OpenBSD 6.8

v9_11 and v9_16 (but not main) fail to build on OpenBSD 6.8 with Clang 10.0.1 (OpenBSD 6.7 with Clang 8.0.1 seems to be fine):

ld: error: undefined symbol: _Unwind_Backtrace
>>> referenced by backtrace.c:120
>>>               backtrace.o:(isc_backtrace_gettrace) in archive ../isc/libisc.a

ld: error: undefined symbol: _Unwind_GetIP
>>> referenced by backtrace.c:101
>>>               backtrace.o:(btcallback) in archive ../isc/libisc.a
clang: error: linker command failed with exit code 1 (use -v to see invocation)
*** Error 1 in target 'sample-request'
ld: error: undefined symbol: _Unwind_Backtrace
>>> referenced by backtrace.c:120
>>>               backtrace.o:(isc_backtrace_gettrace) in archive ../isc/libisc.a

ld: error: undefined symbol: _Unwind_GetIP
>>> referenced by backtrace.c:101
>>>               backtrace.o:(btcallback) in archive ../isc/libisc.a
clang: error: linker command failed with exit code 1 (use -v to see invocation)
*** Error 1 in target 'resolve'
ld: error: undefined symbol: _Unwind_Backtrace
>>> referenced by backtrace.c:120
>>>               backtrace.o:(isc_backtrace_gettrace) in archive ../isc/libisc.a

ld: error: undefined symbol: _Unwind_GetIP
>>> referenced by backtrace.c:101
>>>               backtrace.o:(btcallback) in archive ../isc/libisc.a
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Configuring with --disable-backtrace makes the build pass.