runtime: tune append crossover on amd64 and 386

Fixes issue 4963.

Sets the append crossover to 0 on intel platforms.

Results for linux/amd64 Core i5 SNB

benchmark                     old ns/op    new ns/op    delta
BenchmarkAppend                     102          104   +1.96%
BenchmarkAppend1Byte                 10           11   +0.92%
BenchmarkAppend4Bytes                15           11  -28.10%
BenchmarkAppend7Bytes                17           12  -32.58%
BenchmarkAppend8Bytes                18           12  -36.17%
BenchmarkAppend15Bytes               24           11  -55.02%
BenchmarkAppend16Bytes               25           11  -56.03%
BenchmarkAppend32Bytes               11           12   +4.31%
BenchmarkAppendStr1Byte               8            9  +13.99%
BenchmarkAppendStr4Bytes             11            9  -17.52%
BenchmarkAppendStr8Bytes             14            9  -35.70%
BenchmarkAppendStr16Bytes            21            9  -55.19%
BenchmarkAppendStr32Bytes            10           10   -5.66%
BenchmarkAppendSpecialCase           49           52   +7.96%

Results for linux/386 Atom(TM) CPU 330 @ 1.60GHz

benchmark                     old ns/op    new ns/op    delta
BenchmarkAppend                     219          218   -0.46%
BenchmarkAppend1Byte                 75           72   -3.44%
BenchmarkAppend4Bytes                92           73  -19.87%
BenchmarkAppend7Bytes               108           74  -31.20%
BenchmarkAppend8Bytes               116           74  -35.95%
BenchmarkAppend15Bytes              162           77  -52.22%
BenchmarkAppend16Bytes              169           77  -54.20%
BenchmarkAppend32Bytes               88           86   -2.38%
BenchmarkAppendStr1Byte              57           59   +3.32%
BenchmarkAppendStr4Bytes             72           59  -17.40%
BenchmarkAppendStr8Bytes             92           60  -34.70%
BenchmarkAppendStr16Bytes           141           63  -54.89%
BenchmarkAppendStr32Bytes            75           73   -2.64%
BenchmarkAppendSpecialCase          270          270   +0.00%

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/12440044