Enable `-Wextra` and `-Wunreachable-code-loop-increment`.
Disable `-Wno-unused-parameter`.

Detects the build warning / breakage fixed by https://swiftshader-review.googlesource.com/c/SwiftShader/+/43989.

Bug: none. Reducing chance of future build breakages.
Change-Id: I3001f0ded71f2f71f817581996cc727e345845a3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43991
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>