lib/crypto: chacha: Consolidate into single module

Consolidate the ChaCha code into a single module (excluding
chacha-block-generic.c which remains always built-in for random.c),
similar to various other algorithms:

- Each arch now provides a header file lib/crypto/$(SRCARCH)/chacha.h,
  replacing lib/crypto/$(SRCARCH)/chacha*.c.  The header defines
  chacha_crypt_arch() and hchacha_block_arch().  It is included by
  lib/crypto/chacha.c, and thus the code gets built into the single
  libchacha module, with improved inlining in some cases.

- Whether arch-optimized ChaCha is buildable is now controlled centrally
  by lib/crypto/Kconfig instead of by lib/crypto/$(SRCARCH)/Kconfig.
  The conditions for enabling it remain the same as before, and it
  remains enabled by default.

- Any additional arch-specific translation units for the optimized
  ChaCha code, such as assembly files, are now compiled by
  lib/crypto/Makefile instead of lib/crypto/$(SRCARCH)/Makefile.

This removes the last use for the Makefile and Kconfig files in the
arm64, mips, powerpc, riscv, and s390 subdirectories of lib/crypto/.  So
also remove those files and the references to them.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250827151131.27733-7-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

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