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
Change-Id: Iad3a3fb97ac8907d0053390ea5ce783bf7132905
Exempt-From-Owner-Approval: build system cleanup