[zircon][gn] Fix $zx/system/ulib/zircon deps from fuzzer environment

The libc++ support changes refactored the GN magic around the vDSO
($zx/system/ulib/zircon) target and broke the semantics for derived
environments of the user environment, i.e. user.fuzzer.  Fix that.

Bug: BLD-430 #comment Fix GN build regression introduced by libc++ support.
Bug: ZX-1751 #comment Fix GN build regression introduced by libc++ support.
Change-Id: I47bd7683e97aedb7b2ab7c8ab05a136cd936ff26