For backward compatibility, existing usages of `//go:linkname` found in
a large open-source code corpus remain supported.
Any new references to standard library internal symbols will be disallowed.

A linker command line flag `-checklinkname=0` can be used to disable
this check, for debugging and experimenting purposes.

<!-- CL 473495 -->
When building a dynamically linked ELF binary (including PIE binary), the

	flag8             bool // use 64-bit addresses in symbol table
	flagInterpreter   = flag.String("I", "", "use `linker` as ELF dynamic linker")
	flagCheckLinkname = flag.Bool("checklinkname", true, "check linkname symbol references")
	FlagDebugTramp    = flag.Int("debugtramp", 0, "debug trampolines")
	FlagDebugTextSize = flag.Int("debugtextsize", 0, "debug text section max size")

	for _, r := range l.objs[goObjStart:] {
		st.preloadSyms(r, hashed64Def)
		st.preloadSyms(r, hashedDef)
		st.preloadSyms(r, nonPkgDef)
	}
	for _, vr := range st.linknameVarRefs {
		l.checkLinkname(vr.pkg, vr.name, vr.sym)
	}
	l.nhashedsyms = len(st.hashed64Syms) + len(st.hashedSyms)
	for _, r := range l.objs[goObjStart:] {
		loadObjRefs(l, r, arch)
	}