[ELF] Move createSyntheticSections from Writer.cpp to SyntheticSections.cpp. NFC

SyntheticSections.cpp is more appropriate. This change enables
elimination of many explicit template instantiations.

Due to `make<SymbolTableSection<ELFT>>(*strtab)` in Arch/ARM.cpp,
we do not remove explicit template instantiations for SymbolTableSection.

Author: MaskRay