Catch the set of host platforms up to the set in llvm ports.
To revision bump as it only effects building the port on platforms that
would fail before.
Upgrade to llvm/clang r150242.
Enable installation of the shared libaray in LLVM.
Add preliminary support for building and installing the gold linker
plugin. No option is avaiable to turn it on (use -DWITH_LTOPLUGIN)
because devel/binutils does not yet build the gold linker.