gcc: Add clang's -Wno-c++11-narrowing where we have gcc's -Wno-narrowing. This fixes some clang buildworld issues. Not sure why it errors without this flag even though no -Werror is set.
gcc80: Make bootstrapping more permissive. To allow boostrapping on OpenBSD/Linux hosts: * provide CROSS_LIBDL knob when needed; * disable use of iconv in support libs; * disable werror in hostlibs.
gcc80: Add pregenerated headers.
gcc80: Bring in makefiles but leave them unhooked. Makefile infrastructure was reused from gcc50 one writen by John Marino.