lib/crypto: chacha: Rename libchacha.c to chacha.c

Rename libchacha.c to chacha.c to make the naming consistent with other
algorithms and allow additional source files to be added to the
libchacha module. This file currently contains chacha_crypt_generic(),
but it will soon be updated to contain chacha_crypt().

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250827151131.27733-6-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>