lib/crypto: curve25519: Consolidate into single module

lib/crypto: curve25519: Consolidate into single module

Reorganize the Curve25519 library code:

- Build a single libcurve25519 module, instead of up to three modules:
  libcurve25519, libcurve25519-generic, and an arch-specific module.

- Move the arch-specific Curve25519 code from arch/$(SRCARCH)/crypto/ to
  lib/crypto/$(SRCARCH)/.  Centralize the build rules into
  lib/crypto/Makefile and lib/crypto/Kconfig.

- Include the arch-specific code directly in lib/crypto/curve25519.c via
  a header, rather than using a separate .c file.

- Eliminate the entanglement with CRYPTO.  CRYPTO_LIB_CURVE25519 no
  longer selects CRYPTO, and the arch-specific Curve25519 code no longer
  depends on CRYPTO.

This brings Curve25519 in line with the latest conventions for
lib/crypto/, used by other algorithms.  The exception is that I kept the
generic code in separate translation units for now.  (Some of the
function names collide between the x86 and generic Curve25519 code.  And
the Curve25519 functions are very long anyway, so inlining doesn't
matter as much for Curve25519 as it does for some other algorithms.)

Link: https://lore.kernel.org/r/20250906213523.84915-11-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>