arch: Fix VecReg container alignement to 128bits view

This Patch will fix the alignment problem that appears sometimes
when we try to create a view of 128 bits over the VecRegContainer
object.

That container is initially created as std::array<uint8_t, SIZE>, so
there is no obligation to be aligned to 16 bytes. This patches forces
all containers to be aligned to 16 bytes.

The problem has been observed in the Jira Issue:
https://gem5.atlassian.net/browse/GEM5-320

Change-Id: Id9fdd427bd7a4dc904edd519f31cc29c5b29c5e6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27968
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/generic/vec_reg.hh

@@ -279,7 +279,8 @@ class VecRegContainer
     static constexpr inline size_t size() { return SIZE; };
     using Container = std::array<uint8_t, SIZE>;
   private:
-    Container container;
+    // 16-byte aligned to support 128bit element view
+    alignas(16) Container container;
     using MyClass = VecRegContainer<SIZE>;
 
   public: