arm: fix v4.10-25-g8497b62 fallout

arm: fix v4.10-25-g8497b62 fallout

Starting with commit v4.10-25-g8497b62,

arm$ ./strace true
pid 1234 stray syscall exit
Segmentation fault

Fix this by updating ARM syscall sanity check.
In particular, get_scno() should not set TCB_INSYSCALL flag because
other code assumes that s_ent is properly initialized when this flag
is set.

* linux/arm/get_scno.c: Check syscall number after fetching.
Do not apply the check to SCNO_IN_RANGE syscalls.
Do not set TCB_INSYSCALL flag.
Extend diagnostics and move it to debug level.