BACKPORT: lib/crypto: blake2s: include as built-in

In preparation for using blake2s in the RNG, we change the way that it
is wired-in to the build system. Instead of using ifdefs to select the
right symbol, we use weak symbols. And because ARM doesn't need the
generic implementation, we make the generic one default only if an arch
library doesn't need it already, and then have arch libraries that do
need it opt-in. So that the arch libraries can remain tristate rather
than bool, we then split the shash part from the glue code.

Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org
Cc: linux-crypto@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

(cherry picked from commit 6048fdcc5f269c7f31d774c295ce59081b36e6f9)

Conflicts:
	arch/x86/crypto/Makefile
	arch/x86/crypto/blake2s-glue.c
	lib/crypto/Kconfig

Change-Id: I6861048dfd30aba6fbab8c5543e852e4edca300e
Signed-off-by: Eric Biggers <ebiggers@google.com>