Fix ed25519-donna with SSP on non-x86.

The only reason 16 byte alignment is required is for SSE2 load and
store operations, so only align datastructures to 16 byte boundaries
when building with SSE2 support.

This fixes builds with GCC SSP on platforms that don't have special
case code to do dynamic stack re-alignment (everything not x86/x86_64).

Fixes bug #16666.

src/ext/ed25519/donna/README.tor

@@ -37,3 +37,6 @@ as of 8757bd4cd209cb032853ece0ce413f122eef212c.
    since the compilation will fail in `ge25519_scalarmult_base_choose_niels`
    on x86_64 targets due to running out of registers.
 
+ * On non-x86 targets, GCC's Stack Protector dislikes variables that have
+   alignment constraints greater than that of other primitive types.
+   The `ALIGN` macro is thus no-oped for all non-SSE2 builds.

src/ext/ed25519/donna/ed25519-donna-portable.h

@@ -144,6 +144,20 @@ static inline void U64TO8_LE(unsigned char *p, const uint64_t v) {
 	#endif
 #endif
 
+/* Tor: GCC's Stack Protector freaks out and produces variable length
+ * buffer warnings when alignment is requested that is greater than
+ * STACK_BOUNDARY (x86 has special code to deal with this for SSE2).
+ *
+ * Since the only reason things are 16 byte aligned in the first place
+ * is for SSE2, only request variable alignment for SSE2 builds.
+ *
+ * See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59674
+ */
+#if !defined(ED25519_SSE2)
+	#undef ALIGN
+	#define ALIGN(x)
+#endif
+
 #include <stdlib.h>
 #include <string.h>