Use __builtin___clear_cache() instead of __clear_cache() to avoid having
to define the function as extern. Fix the following warning:

| In file included from qemu/cpus.c:34:
| qemu/exec-all.h: In function 'tb_set_jmp_target1':
| qemu/exec-all.h:208: error: nested extern declaration of '__clear_cache'
| make[1]: *** [cpus.o] Error 1
| make: *** [subdir-i386-softmmu] Error 2

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>