add support for rr's new elapsed-time command

[cs01]

In a recent pull request, a new gdb command was added to rr: elapsed-time.

Investigate whether this command support gdb's machine interface (@WesleyYue do you know?), and if it does, use its values along the x-axis when plotting variables in gdbgui (to view value vs. time in the plot).

[WesleyYue]

I don't have the answer off the top of my head since I'm not familiar with GDB's machine interface. I'm in the middle of my final exams right now but I can take a look at it next week!