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: Use sscratch for per-core kernel stack

When SMP is enabled, the sscratch register is used to
contain the per-core kernel stack instead of the current
running thread. We get the current running thread from the
top of the kernel stack.

RISCV: Remove hartid and DTB parsing from the boot code.

RISC-V: Disable relaxation when loading gp

[SELFOUR-1156] RISC-V Port

Experimental release that supports both RV32 and RV64