riscv: make runtime const not usable by modules

riscv: make runtime const not usable by modules

Similar as commit 284922f4c563 ("x86: uaccess: don't use runtime-const
rewriting in modules") does, make riscv's runtime const not usable by
modules too, to "make sure this doesn't get forgotten the next time
somebody wants to do runtime constant optimizations". The reason is
well explained in the above commit: "The runtime-const infrastructure
was never designed to handle the modular case, because the constant
fixup is only done at boot time for core kernel code."

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://patch.msgid.link/20260221023731.3476-1-jszhang@kernel.org
Signed-off-by: Paul Walmsley <pjw@kernel.org>