cmd/link: do size fixups after symbol references are loaded

When we do a size fixup, we need to clone the symbol to an
external symbol so we can modify it. This includes cloning the
relocations, which includes resolving the relocations. If the
symbol being fixed has a relocation referencing a non-Go symbol,
that symbol has not yet been created, it will be resolved to an
empty symbol. Load the references first, so the referenced symbol,
even if it is a non-Go symbol, exists.

Fixes #74537.

Change-Id: I81525bd7c3e232b80eefeb0f18e13ba5331e1510
Reviewed-on: https://go-review.googlesource.com/c/go/+/687315
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/link/internal/loader/loader.go

@@ -2298,10 +2298,6 @@ func (l *Loader) LoadSyms(arch *sys.Arch) {
 		st.preloadSyms(r, hashedDef)
 		st.preloadSyms(r, nonPkgDef)
 	}
-	for _, sf := range l.sizeFixups {
-		pp := l.cloneToExternal(sf.sym)
-		pp.size = int64(sf.size)
-	}
 	for _, vr := range st.linknameVarRefs {
 		l.checkLinkname(vr.pkg, vr.name, vr.sym)
 	}
@@ -2309,6 +2305,10 @@ func (l *Loader) LoadSyms(arch *sys.Arch) {
 	for _, r := range l.objs[goObjStart:] {
 		loadObjRefs(l, r, arch)
 	}
+	for _, sf := range l.sizeFixups {
+		pp := l.cloneToExternal(sf.sym)
+		pp.size = int64(sf.size)
+	}
 	l.values = make([]int64, l.NSym(), l.NSym()+1000) // +1000 make some room for external symbols
 	l.outer = make([]Sym, l.NSym(), l.NSym()+1000)
 }