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: Ie87f6ff4290d18e099e1be92093b86f743296563
Exempt-From-Owner-Approval: build system cleanup