kernel: remove dead net_ping_test(), ring3 flag; fix CROSS toolchain override

kernel: remove dead net_ping_test(), ring3 flag; fix CROSS toolchain override

- Remove net_ping_test() declaration from include/net.h and gut the
  implementation in src/net/net_ping.c (~190 lines of dead code).
  ICMP ping is now tested in userspace via fulltest I7b (SOCK_RAW).
  File kept as empty placeholder for Makefile wildcard enumeration.

- Remove 'ring3' from known_flags in src/kernel/cmdline.c — the
  ring3 boot test was removed in a prior session and no code
  consumes this flag anymore.

- Add 'override' directive to CC/AS/LD in the CROSS block of the
  Makefile, ensuring the cross-toolchain is forced even if the
  variables are set in the environment or on the make command line.