trivial: Fix compiler warnings

- Remove unused variables
- Conditionally define functions that are conditionally used
- Add default cases to non-exhaustive switch statements
- Correct printf modifiers
- Correct type mismatches
- Initialize variables to prevent uninitialized usages
- Insert missing return statements

Trivial: style fix

make the style checker happy

libsel4camkes: Add aarch64 support

- __NR_pause isn't a valid syscall on aarch64, so we don't add it to the
syscall table
- We modify the entrypoint assembly from aarch32 to work on aarch64.