Use stubs for system_dynamic_deps

system libraries (libc, libm, libdl) belong to the runtime apex api
domain. platform and other apexes should link against stubs and not impl
of these libraries. Update bp2build so that both apex/non_apex picks
stubs. These axes need to be more granular in the future, but for now
this works because
- non_apex axis (platform) is a separate api domain than the runtime apex
- com.android.runtime which matches apex axis has not been allowlisted
  yet.

Test: TH
Change-Id: I43f429a90875f30d176db17b144dd51674ec8adf