sim: Trap into GDB instead of panicking on SEGV

When a segfault happens in the guest, report a SEGV trap to GDB (if
there is one attached) instead of bailing out immediately.

The obvious use-case for this, is the ability to debug guest crashes
in GDB in the standard manner.

The less-trivial use-case is for development of software in an
incomplete software stack (cf. Aarno-Engblom's "Virtual Platforms"
pp.105 et seq.)  One particular example is Ingalls-Miranda simulation of
JIT compilers, where the VM's address space may be split between the
simulated and the real machine: in this case, GDB traps facilitate the
transparent illusion of an unbroken address space.

Change-Id: I9072ed5f6474e05e9a99dc42ae5754be28121355
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44685
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/null/remote_gdb.hh

@@ -47,6 +47,7 @@ class BaseRemoteGDB
 
     bool breakpoint() { return false; }
     void replaceThreadContext(ThreadContext *tc) {}
+    bool trap(int type) { return true; }
 
     virtual ~BaseRemoteGDB() {}
 };