CMakeLists: Enabled more clang warnings.
CMakeLists: Enabled more clang warnings.

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 <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Antonio Maiorano <86d87a6550afb6b07035c9743fa181900589f7e0@google.com>
Tested-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>

bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,google/swiftshader