Local Dynamic uses _TLS_MODULE_BASE_@tlsdesc instead of a bunch of
foo@tlsdesc for each symbol. _TLS_MODULE_BASE_ is a special symbol that
makes _TLS_MODULE_BASE_@tpoff = 0.
For simplicity, synthesize _TLS_MODULE_BASE_ for all targets (GNU linkers
synthesize this symbol for targets that support TLSDESC).
Change the signature of getTlsTpOffset() to special case _TLS_MODULE_BASE_.