Use internal stdint and stddef

This changes sel4runtime to use the internal minimal stdint and stddef
removing the dependency for those files from an external libc.

This also removes the imports and dependencies for stdbool, string, and
assert.

Signed-off-by: Curtis Millar <curtis@curtism.me>

Update licence dates.

Copy memcopy and memset for musl libc.

memset and memcopy have been taken from our fork of the musl libc
library. Appropriate copyright has been added to each file.

Licenses.

Minimise external dependencies.

At the momement, it appears that we will need to rely on at least _some_
C library, although it doesn't matter which.