bp2build: avoid adding deps on system_shared_libs temporarily.

At least until libc and libm builds, which are the common values in
system_shared_libs. We don't have a way to break the cycle of:

libstdc++.so (system_shared_libs) -> libc.so -> ...(whole_static_libs) -> libstdc++.a (system_shared_libs) -> []

because bp2build just sees it as:

libstdc++ (system_shared_libs) -> libc -> (whole_static_libs) -> libstdc++ (system_shared_libs) -> libc -> ...

therefore creating a cycle.

This CL punts the problem for now by ignoring system_shared_libs in the cc deps mutator, with a TODO.

Test: TH

Bug: b/186024507
Bug: b/186489250
Change-Id: Id4744593af39b7eb3b31418c8afaeeac99a0ab5f