riscv: Change code model of module to medany to improve data accessing

All the loaded module locates in the region [＆_end-2G，VMALLOC_END] at
runtime, so the distance from the module start to the end of the kernel
image does not exceed 2GB. Hence, the code model of the kernel module can
be changed to medany to improve the performance data access.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>