libobjc2: use libBlocksRuntime from libdispatch

[hmelder]

Complexity of the Blocks Runtime in libdispatch is lower and only copies the block descriptor not the body. This means we do not need to mprotect a page as -W- and then R-X to execute copied instructions.

[hmelder]

Because of this, there is no need for a thunk written in assembly to jump to the block body residing in the mprotected page.

[triplef]

Sounds great! Builds are currently failing with:

CMake Error at CMakeLists.txt:252 (message):
  libBlocksRuntime does not contain _Block_use_RR2().  Enable
  EMBEDDED_BLOCKS_RUNTIME to use the built-in blocks runtime.

[hmelder]

I know. The header is not in the search path. So this is just a CMake Configuration issue :)