Check that the linker features from Darwin ld64 for marking a symbol as
undefined (in the context of two-level namespace, it means that it can
be looked up in other namespaces) are usable. If they are not, the
static module bank is disabled since it depends on either static linkage
(which is obviously not affected by this option) or more generally the
ability to look up the static module bank symbol in a different linkage
unit/namespace.

In particular, they are currently not usable when enabling bitcode
through LDFLAGS+=" -fembed-bitcode" and compiling dynamically. On tvOS,
bitcode is mandatory, so supporting compiling with bitcode but without
the static module bank seems a rather good compromise given that it was
disabled anyway previously.

In static builds (the current shipping target for iOS and tvOS), the
static module bank stays enabled by the !HAVE_DYNAMIC_PLUGINS check.