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: I19474b84d4afa1b678cd4116a3352fcf3cca9e84
Exempt-From-Owner-Approval: build system cleanup