machine: don't inline RTT WriteByte everywhere

With `-opt=2`, WriteByte gets inlined everywhere a println statement
exists. This blows up binary size for very little gain. In my case, the
binary size roughly doubled. Instead, don't inline it so that the binary
size remains somewhat reasonable. This might slow down WriteByte a tiny
bit, but likely not by any significant amount.

src/machine/serial-rtt.go

@@ -123,6 +123,12 @@ func (b *rttBuffer) buffered() int {
 	return int((writeOffset - readOffset) % rttBufferSizeDown)
 }
 
+// Write a single byte to the RTT output buffer.
+//
+// This method is set to not be inlined, to avoid blowing up binary size as a
+// result of inlining writeByte everywhere a println exists.
+//
+//go:noinline
 func (s *rttSerial) WriteByte(b byte) error {
 	s.buffersUp[0].writeByte(b)
 	return nil