[TSan] Omit vfork interceptor iOS simulator runtime

`_vfork` moved from libsystem_kernel.dylib to libsystem_c.dylib as part
of the below changes.  The iOS simulator does not actually have
libsystem_kernel.dylib of its own, it only has the host Mac's.  The
umbrella-nature of Libsystem makes this movement transparent to
everyone; except the simulator! So when we "back deploy", i.e., use the
current version of TSan with an older simulator runtime then this symbol
is now missing, when we run on the latest OS (but an older simulator
runtime).

Note we use `SANITIZER_IOS` because usage of vfork is forbidden on iOS
and the API is completely unavailable on watchOS and tvOS, even if this
problem is specific to the iOS simulator.

Caused by:
rdar://74818691 (Shim vfork() to fork syscall on iOS)
rdar://76762076 (Shim vfork() to fork syscall on macOS)

Radar-Id: rdar://8634734