Remove default libraries

libdl is part of system_shared_libs now. -ldl -lpthread -lm are now defaults
for host_ldlibs on Linux and Darwin. -lrt is a default for host_ldlibs on
Linux.

Test: m host
Test: mmma bionic
Change-Id: I966e2f88c24fba5e412bee6b6382045a2026a8e4