Members of classes that are declared as static constexpr are not defined
as part of their declaration. Thus they must be defined elsewhere. And
they can't be defined in the header file, or there will be multiple
definitions for each file that includes the header.

So define these contexprs in Constants.cpp

Bug: b/177220369
Change-Id: I9ffc63867948bc72061ddb8b0b586a46d840196c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51628
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Sean Risser <srisser@google.com>
Commit-Queue: Sean Risser <srisser@google.com>