[mono] Fix building multiple os/arch combinations in the same working directory ...

[mono] Fix building multiple os/arch combinations in the same working directory (#39970)

After adding the ICU shim code to the runtime we hit an issue when e.g. compiling for "desktop" mono in a working directory that already had Browser wasm artifacts in it.
This is due to automake putting the intermediate compilation artifacts at the same location where a file was referenced from.

This means that e.g. when using `../../../libraries/Native/Unix/System.Globalization.Native/pal_icushim.c` in the Makefile.am we'd get the intermediaries in `artifacts/obj/mono/libraries/` instead of somewhere in `artifacts/obj/mono/OSX.x64.Debug`.

Later on we'd get the following error because it was reusing the existing .o file from another architecture:

```
Undefined symbols for architecture x86_64:
  "_gPalGlobalizationNative", referenced from:
    _c_qcalls in libmonoruntimesgen.a(libmonoruntimesgen_la-native-library-qcall.o)
```

Fix this by symlinking the source files into the build directory.