Work around riscv buildworld failure where it cannot link the final
clang binary, with:

ld: error: undefined symbol: llvm::EnableABIBreakingChecks

referenced by PlistDiagnostics.cpp

PlistDiagnostics.o:(.sdata+0x0) in archive /usr/obj/usr/src/riscv.riscv64/lib/clang/libclang/libclang.a

[... many more like this ...]