selftests/bpf: Add missing system defines for mips

Update get_sys_includes in Makefile with missing MIPS-related definitions
to fix many, many compilation errors building selftests/bpf. The following
added defines drive conditional logic in system headers for word-size and
endianness selection:

  MIPSEL, MIPSEB
  _MIPS_SZPTR
  _MIPS_SZLONG
  _MIPS_SIM, _ABIO32, _ABIN32, _ABI64

Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/f3cfceaf5299cdd2ac0e0a36072d6ca7be23e603.1721692479.git.tony.ambardar@gmail.com