base: improve gdb exception information

Even it is an unknow exception for GDB, we still can try to cast it
to std::exception and print some information from `what()`.

Change-Id: Ie0cdc978eb1b3e130bc2153eae99025dcd0109f7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48825
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/remote_gdb.cc

@@ -557,6 +557,8 @@ BaseRemoteGDB::trap(ContextID id, int signum)
             send("");
         } catch (CmdError &e) {
             send(e.error);
+        } catch (std::exception &e) {
+            panic("Unrecognzied GDB exception: %s", e.what());
         } catch (...) {
             panic("Unrecognzied GDB exception.");
         }