Re-enable musl-defined wrappers

Re-enable musl-defined wrappers

In the previous version, wrappers over system calls (e.g. `read ()`)
were defined in syscall shim layer. Since we want to maintain musl as
vanilla as possible, we have re-introduced these wrappers.

Signed-off-by: Dragos Iulian Argint <dragosargint21@gmail.com>
Reviewed-by: Sergiu Moga <sergiu.moga@protonmail.com>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
Approved-by: Simon Kuenzer <simon@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #5