[MIPS] Reimplement syscall to invoke the system call directly

Some MIPS kernels do not correctly restart interrupted system calls that
have been invoked using the indirect syscall (NR_syscall).

The simplest workaround is to handle the indirection in userland and then
call the required system call directly.

Change-Id: I8385399621529db9a52b463c96925f6decaaca30