This was being allocated with no particular alignment, causing spurious crashes when vectors were loaded or stored in the stack frame.

Bug: b/135691587
Change-Id: I0240fdeb08df17dbbc156e508013376e15c514b4
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33211
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>