[Bug binutils/27397] New: CET autodetection fails configure on i486 and i586 targets

[slyfox at inbox dot ru]

https://sourceware.org/bugzilla/show_bug.cgi?id=27397

            Bug ID: 27397
           Summary: CET autodetection fails configure on i486 and i586
                    targets
           Product: binutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: binutils
          Assignee: unassigned at sourceware dot org
          Reporter: slyfox at inbox dot ru
                CC: hjl.tools at gmail dot com
  Target Milestone: ---

This is a forward of downstream bugs:
    https://bugs.gentoo.org/760926
    https://bugs.gentoo.org/770061

https://bugs.gentoo.org/760926 is slightly more detailed. There user uses j6-3
machine (almst, but not quite i686). As a result configure detect CET support
by assembler but test crashes at runtime failing ./configure:

```
configure:5391: i486-pc-linux-gnu-gcc -o conftest -pipe -O2 -march=k6-3 -m3dnow
-mmmx -fomit-frame-pointer -ftree-vectorize -mno-sse -mno-sse2 -mno-sse3
-mno-ssse3 -mno-sse4.1 --param l1-cache-size=32 --param l1-cache-line-size=32
--param l2-cache-size=256 -mtune=k6 -fcf-protection=none   -Wl,-O1
-Wl,--as-needed -Wl,-z,ibt,-z,shstk conftest.c  >&5
configure:5391: $? = 0
configure:5391: ./conftest

/var/tmp/portage/sys-devel/gdb-10.1/work/gdb-10.1/libiberty/configure: line
1624:   853 Illegal instruction     ./conftest$ac_exeext
configure:5391: $? = 132
configure: program exited with status 132

configure:5401: error: Intel CET must be enabled on Intel CET enabled host
```

Looks like the only way to get binutils and gdb on old machines is to pass
--disable-cet explicitly to ./configure. Is it an intended behaviour?

Gentoo packages binutils-gdb in a few flavours:
- binutils
- gdb
- binutils-libs
- binutils-hppa64
and we need to duplicate the --disable-cet everywhere.

I ended up disabling CET for all architectures including x86_64 out of
laziness. Would be nice if ./configure picked a better default.

I suggest tweaking upstream configure to silently disable the features instead
of failing configure at least on non-x86_64 to make packaging slightly
smoother.

WDYT?

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