clang: build with MinSizeRel

clang: build with MinSizeRel

This is the recommended build configuration upstream unless speed is the
ultimate goal. They say it's almost as fast as Release (-O3) but a lot
smaller on disk:

  clang-libclang-cpp: PKGSIZE changed from 71729568 to 49368816 (-31%)
  clang-libllvm: PKGSIZE changed from 83015559 to 56662823 (-32%)
  clang-tidy: PKGSIZE changed from 15861679 to 9956175 (-37%)
  clang-tools: PKGSIZE changed from 6865635 to 5374691 (-22%)
  clang: PKGSIZE changed from 29693308 to 22697548 (-24%)
  libclang: PKGSIZE changed from 40087018 to 26966218 (-33%)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>