[llvm-mt] Do not build llvm-mt if not functional (#134631)

llvm-mt requires libxml2 to work, so do not even build it without
libxml2.

CMake 3.31 and later prefer llvm-mt.exe over Microsoft's mt.exe if
available and using clang-cl.exe as CMAKE_CXX_COMPILER. When CMake picks
up llvm-mt.exe without libxml2, any build will fail with the message
```
llvm-mt: error: no libxml2
```

Any test except `--help` already uses `REQUIRES: libxml2`. There is no
point in having a non-functional executable. Not building llvm-mt.exe
will force CMake to use Microsoft's `mt.exe` instead.

Fixes: #134237

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