riscv: Move cpufeature.h macros into their own header

asm/cmpxchg.h will soon need riscv_has_extension_unlikely() macros and
then needs to include asm/cpufeature.h which introduces a lot of header
circular dependencies.

So move the riscv_has_extension_XXX() macros into their own header which
prevents such circular dependencies by including a restricted number of
headers.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Andrea Parri <parri.andrea@gmail.com>
Link: https://lore.kernel.org/r/20241103145153.105097-2-alexghiti@rivosinc.com


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