[RISCV] Define __riscv_cmodel_medlow and __riscv_cmodel_medany correctly

RISC-V LLVM was only implement small/medlow code model, so it defined
__riscv_cmodel_medlow directly without check.

Now, we have medium/medany code model in RISC-V back-end, it should
define according the actually code model.

Reviewed By: lewis-revill

Differential Revision: https://reviews.llvm.org/D67065

llvm-svn: 372078