net: use IndexByte implementation from runtime package

In net/parse.go we reimplement bytes.IndexByte and strings.IndexByte,
However those are implemented in runtime/$GOARCH_asm.s.
Using versions from runtime should provide performance advantage,
and keep the same code together.

Change-Id: I6212184bdf6aa1f2c03ce26d4b63f5b379d8ed0c
Reviewed-on: https://go-review.googlesource.com/15953


Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>