This change applies the same solution as applied to the X86 backends;
defining the instruction opcodes as static constexpr members in the
header.

This was tested locally by building all the backend source files.

Bug: b/192890685
Bug: chromium:604888
Change-Id: I6a70cb7cef1ad7bc56f0ea076b7ed5f4d07de5a3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/55508Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>