Update the host toolchains to be prefixed with their target name and use
a slightly more compact style to define the toolchain scopes.
Change-Id: Ib0dae237299e74c6c48cd4c35869fd215563a7bf
Sets up clang and gcc based toolchains for all host target builds.
Defaults to using Clang for linux/mac and GCC for windows.
Change-Id: I1a15d98581cbf886fbb86321f2d7dabb0dd43cf1