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 broken and untested benchmark stubs

RISCV: Place TODOs in the source

[SELFOUR-1156] RISC-V Port

Experimental release that supports both RV32 and RV64