RISC-V: Export some expected symbols for modules

RISC-V: Export some expected symbols for modules

These are the ones needed by current allmodconfig, so add them instead
of everything other architectures are exporting -- the rest can be
added on demand later if needed.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>