[IA64] Rationalize kernel mode alignment checking

Itanium processors can handle some misaligned data accesses. They
also provide a mode where all such accesses are forced to trap. The
kernel was schizophrenic about use of this mode:

* Base kernel code ran in permissive mode where the only traps
generated were from those cases that the h/w could not handle.
* Interrupt, syscall and trap code ran in strict mode where all
unaligned accesses caused traps to the 0x5a00 unaligned reference
vector.

Use strict alignment checking throughout the kernel, but make
sure that we continue to let user mode use more relaxed mode
as the default.

Signed-off-by: Tony Luck <[e-mail]>

arch/ia64/kernel/entry.S

···
 END(prefetch_stack)
 
 GLOBAL_ENTRY(kernel_execve)
+	rum psr.ac
 	mov r15=__NR_execve			// put syscall number in place
 	break __BREAK_SYSCALL
 	br.ret.sptk.many rp

arch/ia64/kernel/head.S

···
 	 * Switch into virtual mode:
 	 */
 	movl r16=(IA64_PSR_IT|IA64_PSR_IC|IA64_PSR_DT|IA64_PSR_RT|IA64_PSR_DFH|IA64_PSR_BN \
-		  |IA64_PSR_DI)
+		  |IA64_PSR_DI|IA64_PSR_AC)
 	;;
 	mov cr.ipsr=r16
 	movl r17=1f