This change allows building new gcc during bootstrap without fear of
pulling in outdated libstdc++.so after g++ switched from bootstrapTools
to freshly built g++.
Noticed when tried to add early bootstrap stage to rebuild `gcc` before
`glibc` is fully untangled from `bootstrapTools` as a failure to built
`binutils`:
ld: dwp.o: in function `__gnu_cxx::new_allocator<gold::Dwp_output_file::Contribution>::allocate(unsigned long, void const*)':
/nix/store/...-gcc-11.3.0/include/c++/11.3.0/ext/new_allocator.h:116: undefined reference to `std::__throw_bad_array_new_length()'
The change survives existing bootstrap and unblockes early `gcc` bootstrap.
The linker scripts no longer contain store paths, so this does nothing. More
importantly, libpthread.so is not longer a linker script on ARM, so the patching
would corrupt it.
On one hand, don't want to pass garbage that affects hash, on the other
hand footguns are bad.
Now, factored out the derivation so only need to pass in what is used.
