Initial UEFI OS target support after the headers. This just defines
enough that stuff might try and compile. Test with:
```
$ cmake -S llvm -B build -G Ninja -DLLVM_RUNTIME_TARGETS=x86_64-unknown-uefi-llvm -DRUNTIMES_x86_64-unknown-uefi-llvm_LLVM_ENABLE_RUNTIMES=libc -DRUNTIMES_x86_64-unknown-uefi-llvm_LLVM_LIBC_FULL_BUILD=true -DCMAKE_C_COMPILER_WORKS=true -DCMAKE_CXX_COMPILER_WORKS=true -DLLVM_ENABLE_PROJECTS="clang;lld" -DCMAKE_BUILD_TYPE=Debug -DLLVM_ENABLE_LIBCXX=true -DLLVM_HOST_TRIPLE=aarch64-unknown-linux-gnu -DLLVM_DEFAULT_TARGET_TRIPLE=x86_64-unknown-uefi-llvm -DCMAKE_INSTALL_LIBDIR=build/target/lib
$ ninja -C build
```
Originated from #120687
This PR simply adds the necessary headers for UEFI which defines all the
necessary types. This PR unlocks the ability to work on other PR's for
UEFI support.