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.

style: consistently attach return type

Add attach-return-type to astyle

ARM: Swap direct use of externally_visible for VISIBLE macro.

This commit fixes one instances where the `VISIBLE` macro was not used to tag a
function with `externally_visible`. The use of this macro makes it easier to
switch to a non-GNU compiler by providing the alternative syntax for this once.

ARM A15: Remove statement with no effect.

This looks to me like a statement to suppress unused function warnings that
accidentally got shuffled inside the ifdef. This change has no effect on
verification.

Add license header

arm: Add work around for errata on some Cortex-A15 boards and move errata checking to C code