[ELF] --orphan-handling=: don't warn/error for unused synthesized sections

This makes --orphan-handling= less noisy.
This change also improves our compatibility with GNU ld.

GNU ld special cases .symtab, .strtab and .shstrtab . We need output section
descriptions for .symtab, .strtab and .shstrtab to suppress:

  <internal>:(.symtab) is being placed in '.symtab'
  <internal>:(.shstrtab) is being placed in '.shstrtab'
  <internal>:(.strtab) is being placed in '.strtab'

With --strip-all, .symtab and .strtab can be omitted (note, --strip-all is not compatible with --emit-relocs).

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D75149