cpu-all.h: fix cpu_get_real_ticks on mips host

Fix cpu_get_real_ticks:
- check should be done on __mips and not __mips_isa_rev
- linux kernels >= 2.6.25 are emulating the 2 needed rdhwr functions
  so it's safe to use rdhwr.

This is better than what's currently in but it doesn't mean it works nicely
Some tests needs to be done imho

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
