MFC r253917:

After r253839, which modifies ld's behaviour to not automatically pull
in needed libraries, change libc++.so into a linker script, so it can
automatically pull in libcxxrt.so.

[Note to stable/9 users: the above statement about ld's behaviour is not
(yet) applicable to the version of ld in 9.x, but this change will help
when building C++ programs with libc++, in combination with the ports
version of ld.]