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.

Modify FinalizeLog syscall

There is no way to know how many entries the syscall log contains
otherwise. One option was iterating through until finding a 0
terminator, however this requires the log to be zeroed before each
trial. This changes the FinalizeLog syscall to return the number of
entries.

Benchmark: move arch-dependent benchmark.c from api/ to new benchmark/ directory