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