arch-riscv: Define AT_RANDOM properly

arch-riscv: Define AT_RANDOM properly

According to the getauxval(3) man page, the AT_RANDOM aux value should
be a pointer to 16 random bytes.  In the initial implementation of
RISC-V, this was based on spike's program stack setup, which copied the
program header table there instead.  This patch changes the
implementation to use the proper 16 random bytes, making it compatible
with some RISC-V programs that use custom linker scripts.

Change-Id: Idaae7f19bf3ed3fd06d293e5e9c0b6f778270eb2
Reviewed-on: https://gem5-review.googlesource.com/6681
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Alec Roelke <ar4jc@virginia.edu>