dist-various-2: Use clang for the UEFI targets

This fixes an issue where the C and asm sources built by compiler_builtins were being compiled as ELF objects instead of PE objects. This wasn't noticed before because it doesn't cause compiler_builtins or rustc to fail to build. You only see a failure when a program is built that references one of the symbols in an ELF object. Compiling with clang fixes this because the `cc` crate converts the UEFI targets into Windows targets that clang understands, causing it to produce PE objects. Note that this requires compiler_builtins >= 0.1.84. Fixes rust-lang#104326

Author: nicholasbishop

src/ci/docker/host-x86_64/dist-various-2/Dockerfile

@@ -61,6 +61,12 @@ ENV \
  AR_i686_unknown_freebsd=i686-unknown-freebsd12-ar \
  CC_i686_unknown_freebsd=i686-unknown-freebsd12-clang \
  CXX_i686_unknown_freebsd=i686-unknown-freebsd12-clang++ \
+ CC_aarch64_unknown_uefi=clang-11 \
+ CXX_aarch64_unknown_uefi=clang++-11 \
+ CC_i686_unknown_uefi=clang-11 \
+ CXX_i686_unknown_uefi=clang++-11 \
+ CC_x86_64_unknown_uefi=clang-11 \
+ CXX_x86_64_unknown_uefi=clang++-11 \
  CC=gcc-8 \
  CXX=g++-8