riscv: Fix crashes with large or frequent mmaps

This patch fixes a bug where increasing the mmap region too much causes
it to run into already-allocated memory, which causes gem5 to fail an
assertion.  Previously, the stack was incorrectly set up such that the
end of the mmap region and the top of the stack were the same address
and both would grow downward.  With this patch, the top of the stack has
been separated from the end of mmap and moved up, and the mmap region
now grows upward instead of downward.

[Rebase to master branch and remove dependencies.]

Change-Id: I7271ff478fff2994f918bc5003a6139b9ba6a520
Reviewed-on: https://gem5-review.googlesource.com/2680
Maintainer: Alec Roelke <ar4jc@virginia.edu>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

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