Subzero: Enable a cmake build.
Subzero: Enable a cmake build.

This just puts the CMakeLists.txt file in place.  A couple other
changes are needed in other repos to make this take effect.

BUG= none
R=dschuff@chromium.org, mtrofin@chromium.org

Review URL: https://codereview.chromium.org/998693003

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