riscv: [Patch 8/5] Added some regression tests to RISC-V

This patch is the eighth patch in a series adding RISC-V to gem5, and
third of the bonus patches to the original series of five. It adds some
regression tests to RISC-V.

Regression tests included:
- se/00.hello
- se/02.insttest (split into several binaries which are not included due
to large size)

The tests added to 00.insttest will need to be build manually; to
facilitate this, a Makefile is included. The required toolchain and
compiler (riscv64-unknown-elf-gcc) can be built from the riscv-tools
GitHub repository at https://github.com/riscv/riscv-tools.

Note that because EBREAK only makes sense when gdb is running or while in
FS mode, it is not included in the linux-rv64i insttest. ERET is not
included because it does not make sense in SE mode and, in fact, causes
a panic by design.

Note also that not every system call is tested in linux-rv64i; of the ones
defined in linux/process.hh, some have been given numbers but not
definitions for the toolchain, or are merely stubs that always return 0. Of
the ones that do work properly, only a subset are tested due to similar
functionality.

Signed-off by: Alec Roelke

Signed-off by: Jason Lowe-Power <jason@lowepower.com>