Fix poly1305 alignment mismatch.

The C Poly1305 code is expecting the state buffer to be aligned to the
same alignment as `double`. However, the Rust code isn't necessarily
aligning it that much. The C code actually works fine without the
alignment hack as I had explicitly adjusted it to handle unaligned
inputs in a previous commit. However, I overlooked the effect that the
`union` of `double` hack.

This was apparently the cause of the segmentation faults that caused
tests to fail with SIGSEGV when building for ARM/Aarch64 with Stable
Rust. It seems that newer versions of Rust must align things more
granularly as the bug wasn't triggered in Rust Nightly.

include/openssl/poly1305.h

@@ -22,8 +22,7 @@ extern "C" {
 #endif
 
 
-typedef union {
-  double align;
+typedef struct {
   uint8_t bytes[512];
 } poly1305_state;