mk: Fix MSVC bootstrapping itself

Now that MSVC support has landed in the most recent nightlies we can now have
MSVC bootstrap itself without going through a GNU compiler first. Unfortunately,
however, the bootstrap currently fails due to the compiler not being able to
find the llvm-ar.exe tool during the stage0 libcore compile. The compiler cannot
find this tool because it's looking inside a directory that does not exist:

    $SYSROOT/rustlib/x86_64-pc-windows-gnu/bin

The `gnu` on this triple is because the bootstrap compiler's host architecture
is GNU. The build system, however, only arranges for the llvm-ar.exe tool to be
available in this location:

    $SYSROOT/rustlib/x86_64-pc-windows-msvc/bin

To resolve this discrepancy, the build system has been modified to understand
triples that are bootstrapped from another triple, and in this case copy the
native tools to the right location.

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