backtrace: Avoid GCC warning on x86-64.

The portable implementation of stack_low(), which before this commit is
used on x86-64, provokes a warning from GCC that cannot be disabled.  We
already have an i386-specific implementation that does not warn; this
commit adds a corresponding implementation for x86-64 to avoid the warning
there too.

lib/backtrace.c

@@ -73,6 +73,10 @@ stack_low(void)
     uintptr_t low;
     asm("movl %%esp,%0" : "=g" (low));
     return low;
+#elif __x86_64__
+    uintptr_t low;
+    asm("movq %%rsp,%0" : "=g" (low));
+    return low;
 #else
     /* This causes a warning in GCC that cannot be disabled, so use it only on
      * non-x86. */