runtime: add support for time.NewTimer and time.NewTicker

This commit adds support for time.NewTimer and time.NewTicker. It also
adds support for the Stop() method on time.Timer, but doesn't (yet) add
support for the Reset() method.

The implementation has been carefully written so that programs that
don't use these timers will normally not see an increase in RAM or
binary size. None of the examples in the drivers repo change as a result
of this commit. This comes at the cost of slightly more complex code and
possibly slower execution of the timers when they are used.

main_test.go

@@ -65,6 +65,7 @@ func TestBuild(t *testing.T) {
 		"stdlib.go",
 		"string.go",
 		"structs.go",
+		"timers.go",
 		"zeroalloc.go",
 	}
 	_, minor, err := goenv.GetGorootVersion(goenv.Get("GOROOT"))
@@ -180,6 +181,14 @@ func runPlatTests(options compileopts.Options, tests []string, t *testing.T) {
 	}
 
 	for _, name := range tests {
+		if options.GOOS == "linux" && (options.GOARCH == "arm" || options.GOARCH == "386") {
+			switch name {
+			case "timers.go":
+				// Timer tests do not work because syscall.seek is implemented
+				// as Assembly in mainline Go and causes linker failure
+				continue
+			}
+		}
 		if options.Target == "simavr" {
 			// Not all tests are currently supported on AVR.
 			// Skip the ones that aren't.
@@ -208,6 +217,11 @@ func runPlatTests(options compileopts.Options, tests []string, t *testing.T) {
 				// CGo does not work on AVR.
 				continue
 
+			case "timers.go":
+				// Doesn't compile:
+				//   panic: compiler: could not store type code number inside interface type code
+				continue
+
 			default:
 			}
 		}