runtime: disable prefetching on 386

runtime: disable prefetching on 386

It doesn't seem to help on modern processors and it makes Go impossible to run
on Pentium MMX (which is the documented minimum hardware requirement.)

Old is with prefetch, new is w/o. Both are compiled with GO386=sse2.
Benchmarking is done on Intel(R) Core(TM) i5-3570K CPU @ 3.40GHz.

name                     old time/op    new time/op    delta
BinaryTree17-4              2.89s ± 2%     2.87s ± 0%    ~     (p=0.061 n=11+10)
Fannkuch11-4                3.65s ± 0%     3.65s ± 0%    ~     (p=0.365 n=11+11)
FmtFprintfEmpty-4          52.1ns ± 0%    52.1ns ± 0%    ~      (p=0.065 n=10+9)
FmtFprintfString-4          168ns ± 0%     167ns ± 0%  -0.48%   (p=0.000 n=8+10)
FmtFprintfInt-4             167ns ± 0%     167ns ± 1%    ~      (p=0.591 n=9+10)
FmtFprintfIntInt-4          295ns ± 0%     292ns ± 0%  -0.99%   (p=0.000 n=9+10)
FmtFprintfPrefixedInt-4     327ns ± 0%     326ns ± 0%  -0.24%  (p=0.007 n=10+10)
FmtFprintfFloat-4           431ns ± 0%     431ns ± 0%  -0.07%  (p=0.000 n=10+11)
FmtManyArgs-4              1.13µs ± 0%    1.13µs ± 0%  -0.37%  (p=0.009 n=11+11)
GobDecode-4                9.36ms ± 1%    9.33ms ± 0%  -0.31%  (p=0.006 n=11+10)
GobEncode-4                7.38ms ± 1%    7.38ms ± 1%    ~     (p=0.797 n=11+11)
Gzip-4                      394ms ± 0%     395ms ± 1%    ~     (p=0.519 n=11+11)
Gunzip-4                   65.4ms ± 0%    65.4ms ± 0%    ~     (p=0.739 n=10+10)
HTTPClientServer-4         52.4µs ± 1%    52.5µs ± 1%    ~     (p=0.748 n=11+11)
JSONEncode-4               19.0ms ± 0%    19.0ms ± 0%    ~      (p=0.780 n=9+10)
JSONDecode-4               59.6ms ± 0%    59.6ms ± 0%    ~      (p=0.720 n=9+10)
Mandelbrot200-4            4.09ms ± 0%    4.09ms ± 0%    ~      (p=0.295 n=11+9)
GoParse-4                  3.45ms ± 1%    3.43ms ± 1%  -0.35%  (p=0.040 n=11+11)
RegexpMatchEasy0_32-4       101ns ± 1%     101ns ± 1%    ~     (p=1.000 n=11+11)
RegexpMatchEasy0_1K-4       796ns ± 0%     796ns ± 0%    ~      (p=0.954 n=10+8)
RegexpMatchEasy1_32-4       110ns ± 0%     110ns ± 1%    ~      (p=0.289 n=9+11)
RegexpMatchEasy1_1K-4       991ns ± 0%     991ns ± 0%    ~      (p=0.784 n=10+8)
RegexpMatchMedium_32-4      131ns ± 0%     130ns ± 0%  -0.42%   (p=0.004 n=11+9)
RegexpMatchMedium_1K-4     41.9µs ± 1%    41.6µs ± 0%    ~      (p=0.067 n=11+9)
RegexpMatchHard_32-4       2.34µs ± 0%    2.34µs ± 0%    ~     (p=0.208 n=11+11)
RegexpMatchHard_1K-4       70.9µs ± 0%    71.0µs ± 0%    ~      (p=0.968 n=9+10)
Revcomp-4                   819ms ± 0%     818ms ± 0%    ~     (p=0.251 n=10+11)
Template-4                 73.9ms ± 0%    73.8ms ± 0%  -0.25%  (p=0.013 n=10+11)
TimeParse-4                 414ns ± 0%     414ns ± 0%    ~     (p=0.809 n=11+10)
TimeFormat-4                485ns ± 0%     485ns ± 0%    ~      (p=0.404 n=11+7)

name                     old speed      new speed      delta
GobDecode-4              82.0MB/s ± 1%  82.3MB/s ± 0%  +0.31%  (p=0.007 n=11+10)
GobEncode-4               104MB/s ± 1%   104MB/s ± 1%    ~     (p=0.797 n=11+11)
Gzip-4                   49.2MB/s ± 0%  49.1MB/s ± 1%    ~     (p=0.507 n=11+11)
Gunzip-4                  297MB/s ± 0%   297MB/s ± 0%    ~     (p=0.670 n=10+10)
JSONEncode-4              102MB/s ± 0%   102MB/s ± 0%    ~      (p=0.794 n=9+10)
JSONDecode-4             32.6MB/s ± 0%  32.6MB/s ± 0%    ~       (p=0.334 n=9+9)
GoParse-4                16.8MB/s ± 1%  16.9MB/s ± 1%    ~     (p=0.052 n=11+11)
RegexpMatchEasy0_32-4     314MB/s ± 0%   314MB/s ± 1%    ~     (p=0.618 n=11+11)
RegexpMatchEasy0_1K-4    1.29GB/s ± 0%  1.29GB/s ± 0%    ~     (p=0.315 n=10+10)
RegexpMatchEasy1_32-4     290MB/s ± 1%   290MB/s ± 1%    ~     (p=0.667 n=10+11)
RegexpMatchEasy1_1K-4    1.03GB/s ± 0%  1.03GB/s ± 0%    ~      (p=0.829 n=10+8)
RegexpMatchMedium_32-4   7.63MB/s ± 0%  7.65MB/s ± 0%    ~     (p=0.142 n=11+11)
RegexpMatchMedium_1K-4   24.4MB/s ± 1%  24.6MB/s ± 0%    ~      (p=0.063 n=11+9)
RegexpMatchHard_32-4     13.7MB/s ± 0%  13.7MB/s ± 0%    ~     (p=0.302 n=11+11)
RegexpMatchHard_1K-4     14.4MB/s ± 0%  14.4MB/s ± 0%    ~      (p=0.784 n=9+10)
Revcomp-4                 310MB/s ± 0%   311MB/s ± 0%    ~     (p=0.243 n=10+11)
Template-4               26.2MB/s ± 0%  26.3MB/s ± 0%  +0.24%  (p=0.009 n=10+11)

Update #12970.

Change-Id: Id185080687a60c229a5cb2e5220e7ca1b53910e2
Reviewed-on: https://go-review.googlesource.com/15999
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>