[WIP] gcc: don't use sysroot, use proper include search path

Rebuild is tricky:
- rebuild mingw-w64-crt
- rebuild mingw-w64-headers
- rebuild mingw-w64-winpthreads
- reinstall all three above
$ export CPATH=$MINGW_PREFIX/include
$ export LIBRARY_PATH=$MINGW_PREFIX/lib
- rebuild mingw-w64-gcc and reinstall it
- rebuild mingw-w64-binutils and reinstall it

Unfortunately, can't rebuild gcc again for some reason:
it cannot find dllcrt2.o when linking in one of stages.