Fix build of dnssd for 32-bit arm

Using libc++ on arm adds libunwind_llvm as a static library.
libunwind_llvm requires libdl, but we were turning it off along with
libm.

Test: lunch full-eng; m dnssd
Change-Id: Icddf0d67b20c311557cc9bdb0f902fece92dd678
1 file changed