[ELF,RISCV] Fix TLSDESC=>IE when there is no TLS section

See the comment in handleTlsRelocation. For TLSDESC=>IE (the TLS symbol
is defined in another DSO), R_RISCV_TLSDESC_{LOAD_LO12,ADD_LO12_I,CALL}
referencing a non-preemptible label uses the `R_RELAX_TLS_GD_TO_LE` code
path.

If there is no TLS section, `getTlsTpOffset` will be called with null
`Out::tlsPhdr`, leading to a null pointer dereference. Since the return
value is used by `RISCV::relocateAlloc` and ignored there, just return
0.

LoongArch TLSDESC doesn't use STT_NOTYPE labels. The `if (..) return 0;`
is a no-op for LoongArch.

This patch is a follow-up to #79239 and fixes some comments.

Pull Request: https://github.com/llvm/llvm-project/pull/98569

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