runtime: special-case append([]byte, string) for small strings
Also make the crossover point an architecture-dependent constant,
although it's the same everywhere for now.

BenchmarkAppendStr1Byte            416          145  -65.14%
BenchmarkAppendStr4Bytes           743          217  -70.79%
BenchmarkAppendStr8Bytes           421          270  -35.87%
BenchmarkAppendStr16Bytes          415          403   -2.89%
BenchmarkAppendStr32Bytes          415          391   -5.78%

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7459044