allow sifive_e to use more RAM
Description of problem:
For users like me that are still learning RISC bare-metal assembly, searching online you will find many tutorials and examples using sifive_e with Qemu, so it is the easy way to get started.

I quickly ran into crashes with my tests because I did not realize that sifive_e is limited to 16K of RAM.
I realize the 16K limit is hard coded so that it matches the real hardware, but that makes it very hard to run a variety of tests.
Additional information:
My fork of Qemu changes sifive_e to allow 256MB.
https://github.com/panjea/qemu/commit/97cb89d778ebe3407a969b8282e2e7adb4be2971