Improve handling of macos xcode toolchain

On MacOS one can have different xcode versions installed. Switching between these versions for OpenJDK builds can be achieved either via switching the active xcode system-wide using xcode-select or by using the configure flag --with-toolchain-path.

In the latter case, however, the required value for the path is neither self-explaining nor well documented. It turns out that the toolchain path needs 2 components, <xcode_path>/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin and <xcode_path>/Contents/Developer/usr/bin to become effective.

To make it more convenient to pick a certain xcode for a certain build, I suggest adding a configure option --with-xcode-path that simply needs the path to the xcode installations, e.g. /Applications/Xcode.app.