Apply system_shared_libs to all variants

Remove the odd semantics of system_shared_libs that ignore the property
for non-bionic variants by always honoring it.  For the common case
of system_shared_libs: [] this is generally correct, a non-bionic
variant of a module that doesn't compile against bionic libc probably
also doesn't want some other libc.  For modules that use something
like system_shard_libs: ["libc"] to link against libc but not libdl
and libm this may cause problems, as it will cause a host varaint to
try to link against the non-existent host "libc" module, but there
aren't very many of those and they can be fixed to use a target.bionic
clause.

Bug: 193559105
Test: m checkbuild
Change-Id: I94454a71144e3e68c7148b8d0bf2d974d2ad2bc6