[dev.cc] runtime: convert basic library routines from C to Go

[dev.cc] runtime: convert basic library routines from C to Go

float.c held bit patterns for special float64 values,
hiding from the real uses. Rewrite Go code not to
refer to those values directly.

Convert library routines in runtime.c and string.c.

LGTM=r
R=r, dave
CC=austin, dvyukov, golang-codereviews, iant, khr
https://golang.org/cl/170330043