Fix cmake scripts and incorrect dst chroma stride.
Fix cmake scripts and incorrect dst chroma stride.

- README.md and cmake did not build MIPS correctly.
  - llvm is case sensitive and wants "Mips".
- Halide's CMakefiles did not build MIPS by default, causing the Android apps to
  not build out of the box.
- Incorrect dst chroma stride caused blit() to crash on devices with
  srcChromaElementStride == 1.


Former-commit-id: e67abb77abc6eea7d04194342414292740c9cc71
Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide