ARC: 64bit RTSC timestamp hardware issue

The 64bit RTSC is not reliable, causing spurious "jumps" in higher word,
making Linux timekeeping go bonkers. So as of now just use the lower
32bit timestamp.

A cleaner approach would have been removing RTSC support altogether as the
32bit RTSC is equivalent to old TIMER1 based solution, but some customers
can use the 32bit RTSC in SMP syn fashion (vs. TIMER1 which being incore
can't be done easily).

A fallout of this is sched_clock()'s hardware assisted version needs to
go away since it can't use 32bit wrappign counter - and we use the
generic jiffies based version.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
ARC: Remove redundant sigdelsetmask() call from sigreturn

Upstream commit 77097ae503 "most of set_current_blocked() callers want
SIGKILL/SIGSTOP removed from set" folded the sigdelsetmask() call into
set_current_blocked() - so we no longe rneed it in arch code.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
ARC: Fix pt_orig_r8 access

Syscall restarting fixes made pt_regs->orig_r8 a short word, which was
not reflected in the assembler code - thus could potentially break gdb
debugging.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>