Remove "ELF Only" from -f*-sections help text

-ffunction-sections and -fdata-sections are well supported by many
object file formats:
- ELF
- COFF
- XCOFF
- wasm
Only MachO ignores this flag.

While here, remove it from -funique-section-names. Wasm honors this
option.

Addresses PR44910.

Reviewed By: hans, aaron.ballman

Differential Revision: https://reviews.llvm.org/D74634