lib/crc32: don't bother with pure and const function attributes

Drop the use of __pure and __attribute_const__ from the CRC32 library
functions that had them.  Both of these are unusual optimizations that
don't help properly written code.  They seem more likely to cause
problems than have any real benefit.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250208024911.14936-4-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>