Content mismatch between clang_rt.builtins built on linux and windows

I built `clang_rt.builtins` for the `armv7em-none-eabi` configuration with both a linux hosted and a windows hosted compiler (both built from the same source). and the version built by the windows hosted compiler is missing a large number of essential builtins. From the cmake script for compiler_rt it looks like the content of the library is decided by the build host rather than the target.

Start by reproducing the armv7em-none-eabi clang_rt.builtins builds with Linux- and Windows-hosted compilers from the same source, then compare their library contents. Inspect the compiler_rt CMake script mentioned in the report to determine why the host affects the selected builtins; done means both host builds contain the required target builtins and a regression test covers the mismatch.

Written by the indexing model from the issue text.