ec/curve25519.c: avoid 2^51 radix on SPARC.

SPARC ISA doesn't have provisions to back up 128-bit multiplications
and additions. And so multiplications are done with library calls
and carries with comparisons and conditional moves. As result base
2^51 code is >40% slower...
Reviewed-by: NTim Hudson <tjh@openssl.org>

crypto/ec/curve25519.c

@@ -12,6 +12,7 @@
 #include <openssl/sha.h>
 
 #if !defined(PEDANTIC) && \
+    !defined(__sparc__) && \
     (defined(__SIZEOF_INT128__) && __SIZEOF_INT128__==16)
 /*
  * Base 2^51 implementation.