[ELF] Move TLS mismatch error from Symbol::replace to postParse

* detect `def_tls.o undef_nontls.o` violation
* place error checking code (checking duplicate symbol) together
* allow `--defsym tls1=tls2 def_tls.o`

As a degraded error checking, `--defsym tls1=42` violation will not be detected.