This cl adds a mechanism so that extension strings can easily be
marked as supported or unsupported base on a boolean value determined
at runtime.

Bug: b/139491466
Change-Id: I4438a482c92d76025f137a7a77f61863abaae9c9
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/53228
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Commit-Queue: Alexis Hétu <sugoi@google.com>