[Pal/Linux-SGX] gdb wrapper: use add-auto-load-safe-path

Previously, the GDB wrapper of Graphene-SGX used "set auto-load
safe-path GDB_SCRIPT". This contradicts newer GDB versions which have a
libpthread-2.27.so-gdb.py script in a different system dir. Thus, our
wrapper overwrote the native GDB safe-path and broke GDB. This commit
ensures that our wrapper only adds paths to safe-path.

Pal/src/host/Linux-SGX/debugger/gdb

@@ -7,4 +7,4 @@ if [ -z "$INSIDE_EMACS" ]; then
 	set -x
 fi
 
-LD_PRELOAD=$GDB_SO gdb -iex "set auto-load safe-path $GDB_SCRIPT" "$@"
+LD_PRELOAD=$GDB_SO gdb -iex "add-auto-load-safe-path $GDB_SCRIPT" "$@"