linker: Only link to debuggerd on Android

Don't link to it when building with bionic for the host.

Also add libasync_safe, which is used by linker_globals.h even when
debuggerd isn't used.

Bug: 31559095
Test: mma
Test: Attempt to build host bionic
Change-Id: I374e2c2c288133875da82de780b27917ca524240