[Hexagon] Move STT_TLS setting from fixELFSymbolsInTLSFixups to getRelocType
The legacy generic code uses `ELFObjectWriter::fixSymbolsInTLSFixups` to
set `STT_TLS` (and use an unnecessary expression walk). The better way
is to do this in `getRelocType`, which I have done for AArch64, PowerPC,
RISC-V, and X86.