RISC-V vDSO build error after LLVM commit 90cebef1411617fc3eedd359bdf00cb44b1c2439

[nathanchance]

After llvm/llvm-project@90cebef, I see:

$ make -skj"$(nproc)" ARCH=riscv LLVM=1 mrproper allmodconfig vdso_prepare
arch/riscv/kernel/vdso/vdso.so.dbg: dynamic relocations are not supported
make[4]: *** [arch/riscv/kernel/vdso/Makefile:78: arch/riscv/kernel/vdso/vdso.so.dbg] Error 1
...