riscv: fix building external modules

riscv: fix building external modules

[ Upstream commit 5a19c7e06236a9c55dfc001bb4d1a8f1950d23e7 ]

When building external modules, vdso_prepare should not be run.  If the
kernel sources are read-only, it will fail.

Fixes: fde9c59aebaf ("riscv: explicitly use symbol offsets for VDSO")
Signed-off-by: Andreas Schwab <schwab@suse.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>