This adds POWERPC specific hooks for scaled time accounting.

POWER6 includes a SPURR register.  The SPURR is based off the PURR register
but is scaled based on CPU frequency and issue rates.  This gives a more
accurate account of the instructions used per task.  The PURR and timebase
will be constant relative to the wall clock, irrespective of the CPU
frequency.

This implementation reads the SPURR register in account_system_vtime which
is only call called on context witch and hard and soft irq entry and exit.
The percentage of user and system time is then estimated using the ratio of
these accounted by the PURR.  If the SPURR is not present, the PURR read.

An earlier implementation of this patch read the SPURR whenever the PURR
was read, which included the system call entry and exit path.
Unfortunately this showed a performance regression on lmbench runs, so was
re-implemented.

I've included the lmbench results here when run bare metal on POWER6.  1st
column is the unpatch results.  2nd column is the results using the below
patch and the 3rd is the % diff of these results from the base.  4th and
5th columns are the results and % differnce from the base using the older
patch (SPURR read in syscall entry/exit path).

                              Base        Scaled-Acct     SPURR-in-syscall
                             Result      Result  % diff    Result % diff
Simple syscall:              0.3086      0.3086  0.0000    0.3452 11.8600
Simple read:                 0.4591      0.4671  1.7425    0.5044 9.86713
Simple write:                0.4364      0.4366  0.0458    0.4731 8.40971
Simple stat:                 2.0055      2.0295  1.1967    2.0669 3.06158
Simple fstat:                0.5962      0.5876  -1.442    0.6368 6.80979
Simple open/close:           3.1283      3.1009  -0.875    3.2088 2.57328
Select on 10 fd's:           0.8554      0.8457  -1.133    0.8667 1.32101
Select on 100 fd's:          3.5292      3.6329  2.9383    3.6664 3.88756
Select on 250 fd's:          7.9097      8.1881  3.5197    8.2242 3.97613
Select on 500 fd's:          15.2659     15.836  3.7357    15.873 3.97814
Select on 10 tcp fd's:       0.9576      0.9416  -1.670    0.9752 1.83792
Select on 100 tcp fd's:      7.248       7.2254  -0.311    7.2685 0.28283
Select on 250 tcp fd's:      17.7742     17.707  -0.375    17.749 -0.1406
Select on 500 tcp fd's:      35.4258     35.25   -0.496    35.286 -0.3929
Signal handler installation: 0.6131      0.6075  -0.913    0.647  5.52927
Signal handler overhead:     2.0919      2.1078  0.7600    2.1831 4.35967
Protection fault:            0.7345      0.7478  1.8107    0.8031 9.33968
Pipe latency:                33.006      16.398  -50.31    33.475 1.42368
AF_UNIX sock stream latency: 14.5093     30.910  113.03    30.715 111.692
Process fork+exit:           219.8       222.8   1.3648    229.37 4.35623
Process fork+execve:         876.14      873.28  -0.32     868.66 -0.8533
Process fork+/bin/sh -c:     2830        2876.5  1.6431    2958   4.52296
File /var/tmp/XXX write bw:  1193497     1195536 0.1708    118657 -0.5799
Pagefaults on /var/tmp/XXX:  3.1272      3.2117  2.7020    3.2521 3.99398

Also, kernel compile times show no difference with this patch applied.

[pbadari@us.ibm.com: Avoid unnecessary PURR reading]
Signed-off-by: Michael Neuling <mikey@neuling.org>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Jay Lan <jlan@engr.sgi.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>