`libstdc++` and a few other libraries are comiled with the options
set in `EXTRA_TARGET_FLAGS`. Normally, this is filled form
`EXTRA_FLAGS` inside of `builder.sh`, from which it inherits its
optimization option. For cross compilers `EXTRA_TARGET_FLAGS` is
set by a dedicated function that does not specify any optimization,
leading to sub-par runtime performance of many C++ programs.
I hate the thing too even though I made it, and rather just get rid of
it. But we can't do that yet. In the meantime, this brings us more
inline with autoconf and will make it slightly easier for me to write a
pkg-config wrapper, which we need.
