Introduce host-c-toolchain, host-cxx-toolchain, and host-libcxx
metacomponents, meant to be used to depend on a generic c/c++/libc++ is
available.
Also introduce gcc-host-toolchain, providing host gcc and binutils.

This variable is needed to make toolchain/host/gcc work with e.g.
Ubuntu and other distros.
Without it crt{1,i,n}.o, Scrt.o and libc.so cannot be found.

GCC 4.9.3 seems to leak `CFLAGS` for the compiler itself into
`CFLAGS_FOR_TARGET`, leading to trying to link `libgcc` (and others)
against legacy-glibc.