riscv32: Use medany C model for modules

When CONFIG_CMODEL_MEDLOW is used it ends up generating riscv_hi20_rela
relocations in modules which are not resolved during runtime and
following errors would be seen

[    4.802714] virtio_input: target 00000000c1539090 can not be addressed by the 32-bit offset from PC = 39148b7b
[    4.854800] virtio_input: target 00000000c1539090 can not be addressed by the 32-bit offset from PC = 9774456d

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>