scons: Simplify backtrace implementation detection.

scons: Simplify backtrace implementation detection.

There are really only two options current, glibc or none. If there's a
working implementation there's no real reason to select none, and if
there isn't there's no other option but to select none.

Instead of building up a list, having a default, and making what option
on the list is selected configurable, boil it down to either using glibc
if that implementation is detected, or warning and using none. Also
merge the "normal" and *BSD versions of the checks to reduce redundancy.

The complexity can be added back in if/when there are other
implementations to choose from.

Change-Id: I27c77996a00018302f4daea40924cf059d5a4323
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40864
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>