[ELF] Move TLS dynamic relocations to postScanRelocations

This temporarily increases sizeof(SymbolUnion), but allows us to mov GOT/PLT/etc
index members outside Symbol in the future.

Then, we can make TLSDESC and TLSGD use different indexes and support mixed
TLSDESC and TLSGD (tested by x86-64-tlsdesc-gd-mixed.s).

Note: needsTlsGd and needsTlsGdToIe may optionally be combined.

Test updates are due to reordered GOT entries.