compiler,runtime: fix multiple definitions of a single function

strings.IndexByte was implemented in the runtime up to Go 1.11. It is
implemented using a direct call to internal/bytealg.IndexByte since Go
1.12.

Make sure we remain compatible with both.

compiler/compiler.go

@@ -820,6 +820,9 @@ func (c *Compiler) parseFunc(frame *Frame) {
 	if c.DumpSSA {
 		fmt.Printf("\nfunc %s:\n", frame.fn.Function)
 	}
+	if !frame.fn.LLVMFn.IsDeclaration() {
+		panic("function is already defined: " + frame.fn.LLVMFn.Name())
+	}
 	if !frame.fn.IsExported() {
 		frame.fn.LLVMFn.SetLinkage(llvm.InternalLinkage)
 		frame.fn.LLVMFn.SetUnnamedAddr(true)