[lld][WebAssembly] Handle TLS variables in Symbol::getVA. NFC

In the shared memory case we can always assume that TLS addresses
are relative to __tls_base.  In the non-shared memory case TLS
variables are absolute, just like normal data addresses.

This simplifies the code in calcNewValue so that TLS relocations
no longer need special handling.

Differential Revision: https://reviews.llvm.org/D112831