Using stlport_static or c++_static requires libdl.

Why? For one, libgcc's unwinder makes use of `dl_iterate_phdr`. Also,
libgabi++ (the home grown C++ RT we use for stlport) uses
`dlopen` for liblog when reporting fatal errors. The LLVM unwinder
which is used by libc++ also uses libdl.

Requiring a dependency on libdl seems less objectionable than
requiring one on liblog. We could always change libgabi++ to  use
syslog instead, but that will only reach logcat for newer devices
(possibly L+, definitely M+).

Requiring libdl seems like the best option here, especially given
that libgcc needs it anyway.

Change-Id: I4acfaf38145c39fc15a76fbb282a46786e5322f1