riscv: Implement benchmark log buffer

Can now perform benchmarks on the kernel using the log buffer to trace
kernel behavior.

Signed-off-by: Curtis Millar <curtis.millar@data61.csiro.au>

risc-v: Implement benchmark timestamp

Implements timestamp function used by benchmark logging utilities.

Signed-off-by: Curtis Millar <curtis.millar@data61.csiro.au>
Signed-off-by: Curtis Millar <curtis@curtism.me>

replacing all ifndef with pargma once

All the kernel header files now use pargma once rather than the ifndef,
as the pre-processed C files do not change while header files
are protected with pargma once. This will also solve any naming issues
caused by ifndef.

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

riscv: remove unused benchmark code

RISCV: Place TODOs in the source

[SELFOUR-1156] RISC-V Port

Experimental release that supports both RV32 and RV64