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

This syncs it with how the UEFI targets are built in dist-various-2.

If clang isn't the C compiler used for the UEFI targets, or if the wrong
`--target` is passed to clang, we will get ELF objects in some
rlibs. This will cause problems at link time when trying to compile a
UEFI program that uses any of those objects. Add a check to the dist
step for UEFI targets that reads each rlib with the `object` crate and
fails with an error if any non-COFF objects are found.