RISC-V: Only provide the single-letter extensions in HWCAP

The recent refactoring led to us leaking some HWCAP bits to userspace
that didn't make much sense.  With any luck we'll have a better scheme
soon, but for now just mask off those bits to avoid polluting userspace.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230202233832.11036-1-palmer@rivosinc.com


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