compress .gnu.debuglto_.debug_* sections if requested

Right now, when using LTO, the intermediate object files do contain
debug info in sections starting with .gnu.debuglto_ prefix and are
not compressed when --compress-debug-sections is used.

It's a mistake and we can save quite some disk space. The following
example comes from tramp3d when the corresponding LTO sections
are compressed with zlib:

$ bloaty tramp3d-v4-v2.o -- tramp3d-v4.o
    FILE SIZE        VM SIZE
 --------------  --------------
   +83%     +10  [ = ]       0    [Unmapped]
 -68.0%    -441  [ = ]       0    .gnu.debuglto_.debug_line
 -52.3%    -759  [ = ]       0    .gnu.debuglto_.debug_line_str
 -62.4% -3.24Ki  [ = ]       0    .gnu.debuglto_.debug_abbrev
 -64.8% -1.12Mi  [ = ]       0    .gnu.debuglto_.debug_info
 -88.8% -4.58Mi  [ = ]       0    .gnu.debuglto_.debug_str
 -27.7% -5.70Mi  [ = ]       0    TOTAL

bfd/ChangeLog:

	* elf.c (_bfd_elf_make_section_from_shdr): Compress all debug
	  info sections.

gas/ChangeLog:

	* write.c (compress_debug): Compress also ".gnu.debuglto_.debug_"
	if the compression algorithm is different from zlib-gnu.