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.

Fix licenses (the rest)

s/D61/DATA61/ in license headers for consistency

aarch64: Initial implementation

aarch64: add preliminary folders and Makefiles