riscv: introduce asm/swab.h

riscv: introduce asm/swab.h

Implement endianness swap macros for RISC-V.

Use the rev8 instruction when Zbb is available. Otherwise, rely on the
default mask-and-shift implementation.

Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Ignacio Encinas <ignacio@iencinas.com>
Link: https://lore.kernel.org/r/20250723-riscv-swab-v6-1-fc11e9a2efc9@iencinas.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>