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.

ARM: Add support to flush L1 caches

Extend the existing seL4_BenchmarkFlushCaches without modifing the usage
of it. Will be used in sel4bench to measure the cold cache performance.

aarch64: adjustments to pass c-parser

This commit updates the aarch64 build to pass the c-parser.

- replace 0b constants with decimal
- remove empty array definition
- replace __uint128_t with uint64_t, and double the array size
- remove variable shadowing

No further verification guarantees are provided for aarch64 code.

style: consistently attach return type

Add attach-return-type to astyle

armv8: Remove unused function

Fix licenses (the rest)

s/D61/DATA61/ in license headers for consistency

aarch64: Initial implementation

aarch64: add preliminary folders and Makefiles