Move ExecuTorch-specific stuff in shim to shim_et (try #3) (#8647)

In preparation for replacing the checked-in files in shim/ with https://github.com/facebook/buck2-shims-meta as a submodule. The portions of this diff that are not a pure move are:

creation of shim_et/BUCK
symlink shim/tools to shim_et/tools so that fbsource//tools
movement of Android toolchains from shim/BUCK to shim_et/BUCK because they are not present in buck2-shims-meta
edits to .buckconfig
edit to Utils.cmake to point at shim_et// instead of shim//
edit to shim_et/README.md (note that shim/README.md will come back when shim/ is a submodule pointing to buck2-shims-meta)