Fix "--only-keep-debug for ELF relocatables" binutils test for compilers which...

Fix "--only-keep-debug for ELF relocatables" binutils test for compilers which add .debug_macro sections to object files.

  PR 30699
  * binutils/testsuite/binutils-all/objcopy.exp (keep_debug_symbols_for_elf_relocatable): Do not add sections containing the string "debug_" to the list of non-debug sections.

(cherry picked from commit b99a9693)