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.

trivial: Fix typo'd CONFIG_DEBUG option

style: consistently attach return type

Add attach-return-type to astyle

RISC-V: handle all faults

This change alters RISC-V fault handling to pass all non-VM faults as
user exceptions. As we cannot guarantee what the hardware will send us
in the scause register, we must handle all possible values.

It also makes fault handling more consistent with other architectures in
c_traps.c.

serial: remove putConsoleChar

putConsoleChar was needless indirection around putDebugChar. Remove
this.

riscv: Add UNUSED tags

These variables are unused in certain build configurations.

riscv: Define explicit functions for registers

Replaces (read|write|set|clear)_csr macros with explicit functions. This provides a boundary
for verification to reason about the operation.

riscv: Remove currently unsupported FPU

riscv: add missing cases to print_format_cause

riscv: use fault constants instead of magic

riscv: move io.c to arch level

where it belongs