Add default_shared_libs property

Building against a locally-built libc besides bionic requires setting
system_shared_libs to avoid circular dependencies, but modules all
over the tree assume that system_shared_libs only affects modules
building against bionic.  Add a new default_shared_libs property
that applies to all modules, which will generally be set in
arch-specific clauses to only affect the desired set of modules.

Bug: 190084016
Test: m checkbuild
Change-Id: Ia2349d84c70e503916f90a5d2702e135248f73df