[mobile_static] Use clang++ for the linker (#3308)

[mobile_static] Use clang++ for the linker (#3308)

In order for our exception handling to work on linux, we need the
unwind-dw2-fde-dip implementation in libgcc to work with our
.o file. This requires that `--eh-frame-hdr` be passed by the linker.

By using clang++ as the linker, we will get this and other flags
necessary for correct interop with c++ libraries.