riscv: Add runtime constant support

Implement the runtime constant infrastructure for riscv. Use this
infrastructure to generate constants to be used by the d_hash()
function.

This is the riscv variant of commit 94a2bc0f ("arm64: add 'runtime
constant' support") and commit e3c92e81 ("runtime constants: add
x86 architecture support").

[ alex: Remove trailing whitespace ]

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250319-runtime_const_riscv-v10-2-745b31a11d65@rivosinc.com


Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>