Work around a GCC VRP bug.

Spotted on:

gcc (SUSE Linux) 5.1.1 20150713 [gcc-5-branch revision 225736]
openSUSE 20151014 (armv7hl)
VERSION = 20151014
CODENAME = Tumbleweed

src/libsodium/crypto_sign/ed25519/ref10/sc_muladd.c

@@ -33,6 +33,7 @@ Output:
   where l = 2^252 + 27742317777372353535851937790883648493.
 */
 
+#pragma GCC optimize ("no-tree-vrp")
 void sc_muladd(unsigned char *s,const unsigned char *a,const unsigned char *b,const unsigned char *c)
 {
   crypto_int64 a0 = 2097151 & load_3(a);