Fix msys2 linking and handling out of tree build directory for source zip creation

The MSYS2 ln command does not create links on windows but in fact does a deep copy of the filesystem that is to be linked. There doesn't seem to be an imminent issue with that currently in OpenJDK head but using MSYS2 in JDK11 updates which includes the graal compiler projects will lead to errors like:

/usr/bin/ln: failed to create symbolic link '/d/a/jdk11u-dev/jdk11u-dev/build/windows-x64/support/src/src/jdk.internal.vm.compiler.management/share/classes/jdk.internal.vm.compiler.management' -> '../../../../../../../../src/jdk.internal.vm.compiler.management/share/classes': File name too long

Here, the reported filenames actually aren't too long but somehow during copying the subtree ln probably encounters files with a name too long for msys.

We could overcome this by setting a Windows file system junction instead of using ln. Maybe that's even better for build performance.

While debugging this I also found that the handling of file relativity to the top directory does not account for a build directory that could reside outside the source tree.