Prefix all external symbols in runtime by runtime·,
to avoid conflicts with possible symbols of the same
name in linked-in C libraries.  The obvious conflicts
are printf, malloc, and free, but hide everything to
avoid future pain.

The symbols left alone are:

	** known to cgo **
	_cgo_free
	_cgo_malloc
	libcgo_thread_start
	initcgo
	ncgocall

	** known to linker **
	_rt0_$GOARCH
	_rt0_$GOARCH_$GOOS
	text
	etext
	data
	end
	pclntab
	epclntab
	symtab
	esymtab

	** known to C compiler **
	_divv
	_modv
	_div64by32
	etc (arch specific)

Tested on darwin/386, darwin/amd64, linux/386, linux/amd64.

Built (but not tested) for freebsd/386, freebsd/amd64, linux/arm, windows/386.

R=r, PeterGo
CC=golang-dev
https://golang.org/cl/2899041