include/api: Migrate to upstream musl headers

Replace include/api headers that are identical with our upstream musl
version with symlinks to the musl headers. Once we have migrated all the
headers, we can make include/api a symlink to musl/include.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
apps: update to latest

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>