riscv: toolchain arguments for a 64-bit build

For now we enforce the medany code model for 64-bit builds as we get
reloc issues otherwise. The instruction set and ABI are also set to
soft-float usage.

The ilp32 ABI is explicitly specified on 32-bit build to make sure
it is not using a wrong default if the same toolchain is used for both
32- and 64-bit builds. The archittecture options are the same as the
SDK's riscv32 toolchain default in that case.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>