crypto: curve25519 - Fix selftest build error

If CRYPTO_CURVE25519 is y, CRYPTO_LIB_CURVE25519_GENERIC will be
y, but CRYPTO_LIB_CURVE25519 may be set to m, this causes build
errors:

lib/crypto/curve25519-selftest.o: In function `curve25519':
curve25519-selftest.c:(.text.unlikely+0xc): undefined reference to `curve25519_arch'
lib/crypto/curve25519-selftest.o: In function `curve25519_selftest':
curve25519-selftest.c:(.init.text+0x17e): undefined reference to `curve25519_base_arch'

This is because the curve25519 self-test code is being controlled
by the GENERIC option rather than the overall CURVE25519 option,
as is the case with blake2s.  To recap, the GENERIC and ARCH options
for CURVE25519 are internal only and selected by users such as
the Crypto API, or the externally visible CURVE25519 option which
in turn is selected by wireguard.  The self-test is specific to the
the external CURVE25519 option and should not be enabled by the
Crypto API.

This patch fixes this by splitting the GENERIC module from ...