riscv: fix error on memory op address overflow

riscv: fix error on memory op address overflow

Previously, if a memory operation referenced an address that caused the
data to wrap around to the beginning of the memory (such as -1 or
0xFFFFFFFFFFFFFFFF), an assert would fail during address translation and
gem5 would crash.  This patch fixes that by checking for such a case in
RISC-V's TLB code and returning a fault from translateData if that would
happen.  Because RISC-V does support unaligned memory accesses, no
checking is performed to make sure that an access doesn't cross a cache
line.

[Update creation of page table fault to use make_shared.]
[Add comment explaining the change and assertion that the memory request
isn't zero size.]

Change-Id: I7b8ef9a5838f30184dbdbd0c7c1655e1c04a9410
Reviewed-on: https://gem5-review.googlesource.com/2345
Maintainer: Alec Roelke <ar4jc@virginia.edu>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>