We cannot copy a file from termux_step_host_build() as that function
will not be called when rebuilding a package (the host built dir is
reused across builds).
Fixes#2764.
This will make things as std::to_string() and other C++ features
work with a modern and supported C++ library.
We package up libc++_shared.so and bump the revision of every C++
using package to make it rebuild against it.
Users who have built C++ using libraries or programs will probably
need to rebuild them if they are linked against Termux-supplied
libraries, as user code was previously linked against gnustl while
the system libraries are now using libc++, and it's not a good idea
to mix C++ standard libraries in a program.
Fredrik Fornwall