runtime: skip TestSegv traceback check on 386

The VDSO (__kernel_vsyscall) is reachable via
asmcgocall(cgo_start_thread) on linux-386, which causes traceback to
throw.

Fixes #49182.
For #50504.

Change-Id: Idb78cb8de752203ce0ed63c2dbd2d12847338688
Reviewed-on: https://go-review.googlesource.com/c/go/+/376656


Reviewed-by: Cherry Mui <cherryyz@google.com>
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>