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: objdump -x out/host/linux-x86/lib64/libunwind.so
Test: nm -a out/host/linux-x86/lib64/libunwind.so | grep __CTOR_END__
Change-Id: I1ae950ef49f741d4df649405449d01f3f16f8ef1