In ui/qt, we patch `linux-clang/qmake.conf` in order to build `qmake`
using the appropriate flags. However, these changes used to get
installed, leading to hardcoding paths from the CI.

This commit removes the offending lines after the install phase.

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.