ec/curve25519.c: resolve regression with Android NDK's arm64 gcc.
Unlike "upstream", Android NDK's arm64 gcc [but not clang] performs
64x64=128-bit multiplications with library calls, which appears to
have devastating impact on performance. [The condition is reduced to
__ANDROID__ [&& !__clang__], because x86_64 has corresponding
assembly module.]
#if defined(X25519_ASM) \
|| ( (defined(__SIZEOF_INT128__) && __SIZEOF_INT128__ == 16) \
- && !defined(__sparc__) )
+ && !defined(__sparc__) \
+ && !(defined(__ANDROID__) && !defined(__clang__)) )
/*
* Base 2^51 implementation.
*/
