arm64: kernel: implement fast refcount checking

This adds support to arm64 for fast refcount checking, as proposed by
Kees for x86 based on the implementation by grsecurity/PaX.

The general approach is identical: the existing atomic_t helpers are
cloned for refcount_t, with the arithmetic instruction modified to set
the PSTATE flags, and one or two branch instructions added that jump to
an out of line handler if overflow, decrement to zero or increment from
zero are detected.

One complication that we have to deal with on arm64 is the fact that
it has two atomics implementations: the original LL/SC implementation
using load/store exclusive loops, and the newer LSE one that does mostly
the same in a single instruction. So we need to clone some parts of
both for the refcount handlers, but we also need to deal with the way
LSE builds fall back to LL/SC at runtime if the hardware does not
support it.

As is the case with the x86 version, the performance delta is in the
noise (Cortex-A57 @ 2 GHz, using LL/SC not LSE), even though the arm64
implementation incorporates an add-from-zero check as well:

perf stat -B -- echo ATOMIC_TIMING >/sys/kernel/debug/provoke-crash/DIRECT

 Performance counter stats for 'cat /dev/fd/63':

      65716.592696      task-clock (msec)         #    1.000 CPUs utilized
                 2      context-switches          #    0.000 K/sec
                 0      cpu-migrations            #    0.000 K/sec
                46      page-faults               #    0.001 K/sec
      131341846242      cycles                    #    1.999 GHz
       36712622640      instructions              #    0.28  insn per cycle
   <not supported>      branches
            792754      branch-misses

      65.736371584 seconds time elapsed

perf stat -B -- echo REFCOUNT_TIMING >/sys/kernel/debug/provoke-crash/DIRECT

      65615.259736      task-clock (msec)         #    1.000 CPUs utilized
                 2      context-switches          #    0.000 K/sec
                 0      cpu-migrations            #    0.000 K/sec
                45      page-faults               #    0.001 K/sec
      131138621533      cycles                    #    1.999 GHz
       43155978260      instructions              #    0.33  insn per cycle
   <not supported>      branches
            779668      branch-misses

      65.616216112 seconds time elapsed

For comparison, the numbers below were captured using CONFIG_REFCOUNT_FULL,
which uses the validation routines implemented in C:

perf stat -B -- echo REFCOUNT_TIMING >/sys/kernel/debug/provoke-crash/DIRECT

 Performance counter stats for 'cat /dev/fd/63':

     104566.154096      task-clock (msec)         #    1.000 CPUs utilized
                 2      context-switches          #    0.000 K/sec
                 0      cpu-migrations            #    0.000 K/sec
                46      page-faults               #    0.000 K/sec
      208929924555      cycles                    #    1.998 GHz
      131354624418      instructions              #    0.63  insn per cycle
   <not supported>      branches
           1604302      branch-misses

     104.586265040 seconds time elapsed

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>