configure: Add workaround for buggy binutils 2.35 [PR98811]

configure: Add workaround for buggy binutils 2.35 [PR98811]

binutils since https://sourceware.org/bugzilla/show_bug.cgi?id=25612
changes from March last year until the
https://sourceware.org/pipermail/binutils/2020-August/112684.html
fix in early August emits incorrect .debug_info when assembling files
with --gdwarf-5.  Instead of emitting proper DWARF 5 .debug_info header,
it emits DWARF 4 .debug_info header with 5 as the dwarf version instead of
4.  This results e.g. in libgcc.a (morestack.o) having garbage in its
.debug_info sections and e.g. libbacktrace during pretty much all libgo
tests fails miserably.

The following patch adds a workaround for that, don't set
HAVE_AS_GDWARF_5_DEBUG_FLAG if readelf can't read the .debug_info back.

Built tested on x86_64-linux against both binutils 2.35 (buggy ones) and
latest binutils trunk, the former with the patch now has DWARF 3
.debug_line and DWARF 2 .debug_info in morestack.o, while the latter
as before correct DWARF 5 .debug_line and .debug_info.

2021-01-25  Jakub Jelinek  <jakub@redhat.com>

	PR debug/98811
	* configure.ac (HAVE_AS_GDWARF_5_DEBUG_FLAG): Only define if
	readelf -wi is able to read the emitted .debug_info back.
	* configure: Regenerated.