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 external/libese
Change-Id: I421b9eed3d9f913b0dd11d8a81fadaf7fe7734ec
Exempt-From-Owner-Approval: build system cleanup