arch: riscv: Introduce code model options

This commit introduces a new Kconfig choice for configuring the code model
used for compilation.

All three code models specified by the RISC-V ELF psABI specification are
supported: medlow, medany and large.

For RV32, the `medlow` code model is always selected.
For RV64, the `large` code model is selected if the SRAM or kernel VM base
address is greater than or equal to 4 GiB; otherwise, the `medany` code
model is selected.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>