Add CROSS_TOOLCHAIN macro select pre seeded external toolchain configuration files
The goal is to provide pre seeded toolchain configurations withing the ports tree
to allow the use of an external toolchain in a simple way:

make CROSS_TOOLCHAIN=powerpc64-gcc TARGET=powerpc TARGET_ARCH=powerpc64 buildworld

This will look for the external toolchain definition in /usr/local/share/mk/powerpc64-gcc.mk
While here add the notion of X_COMPILER_TYPE to the external toolchain framework to allow
to deal with differences between gcc and clang in regards of cross building