GH-113464: A JIT backend for tier 2 (GH-113465)

Add an option (--enable-experimental-jit for configure-based builds
or --experimental-jit for PCbuild-based ones) to build an
*experimental* just-in-time compiler, based on copy-and-patch
(https://fredrikbk.com/publications/copy-and-patch.pdf).

See Tools/jit/README.md for more information, including how to install the required build-time tooling. Merry JIT-mas! ;)