[lld-macho][nfc] Convert the mock libSystem.tbd to TBDv4

It doesn't seem like TBDv3 allows for specifying multiple platforms, so I'm
upgrading us to TBDv4. (We need to support multiple platforms in order to test
that we can handle zippered dylibs; that functionality will be added in an
upcoming diff.)

Differential Revision: https://reviews.llvm.org/D101953