riscv: lib: Introduce has_fast_unaligned_access()

Create has_fast_unaligned_access to avoid needing to explicitly check
the fast_misaligned_access_speed_key static key.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Evan Green <evan@rivosinc.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Tested-by: Samuel Holland <samuel.holland@sifive.com>
Link: https://lore.kernel.org/r/20240308-disable_misaligned_probe_config-v9-1-a388770ba0ce@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>