[Binary] Hard-code the alignment of the offloading binary

Summary:
We previously used `alignof` to get the necessary alignment of the
binary header. However this was different on 32-bit platforms and caused
a few tests to fail because of it. This patch just changes this to be a
hard-coded constant of 8.

llvm/include/llvm/Object/OffloadBinary.h

@@ -80,7 +80,7 @@ public:
   /// Serialize the contents of \p File to a binary buffer to be read later.
   static std::unique_ptr<MemoryBuffer> write(const OffloadingImage &);
 
-  static uint64_t getAlignment() { return alignof(Header); }
+  static uint64_t getAlignment() { return 8; }
 
   ImageKind getImageKind() const { return TheEntry->TheImageKind; }
   OffloadKind getOffloadKind() const { return TheEntry->TheOffloadKind; }