crypto: lib/curve25519 - work around Clang stack spilling issue

Arnd reports that the 32-bit generic library code for Curve25119 ends
up using an excessive amount of stack space when built with Clang:

  lib/crypto/curve25519-fiat32.c:756:6: error: stack frame size
      of 1384 bytes in function 'curve25519_generic'
      [-Werror,-Wframe-larger-than=]

Let's give some hints to the compiler regarding which routines should
not be inlined, to prevent it from running out of registers and spilling
to the stack. The resulting code performs identically under both GCC
and Clang, and makes the warning go away.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>