crypto: x86/chacha - expose SIMD ChaCha routine as library function

Wire the existing x86 SIMD ChaCha code into the new ChaCha library
interface, so that users of the library interface will get the
accelerated version when available.

Given that calls into the library API will always go through the
routines in this module if it is enabled, switch to static keys
to select the optimal implementation available (which may be none
at all, in which case we defer to the generic implementation for
all invocations).

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>