machine: add __tinygo_spi_tx function to simulator

This is much, _much_ faster than __tinygo_spi_transfer which can only
transfer a single byte at a time. This is especially important for
simulated displays.

I've already implemented the browser side of this on the playground and
have used this patch for local testing where it massively speeds up
display operations.

src/machine/machine_generic.go

@@ -66,12 +66,47 @@ func (spi SPI) Transfer(w byte) (byte, error) {
 	return spiTransfer(spi.Bus, w), nil
 }
 
+// Tx handles read/write operation for SPI interface. Since SPI is a syncronous write/read
+// interface, there must always be the same number of bytes written as bytes read.
+// The Tx method knows about this, and offers a few different ways of calling it.
+//
+// This form sends the bytes in tx buffer, putting the resulting bytes read into the rx buffer.
+// Note that the tx and rx buffers must be the same size:
+//
+//	spi.Tx(tx, rx)
+//
+// This form sends the tx buffer, ignoring the result. Useful for sending "commands" that return zeros
+// until all the bytes in the command packet have been received:
+//
+//	spi.Tx(tx, nil)
+//
+// This form sends zeros, putting the result into the rx buffer. Good for reading a "result packet":
+//
+//	spi.Tx(nil, rx)
+func (spi SPI) Tx(w, r []byte) error {
+	var wptr, rptr *byte
+	var wlen, rlen int
+	if len(w) != 0 {
+		wptr = &w[0]
+		wlen = len(w)
+	}
+	if len(r) != 0 {
+		rptr = &r[0]
+		rlen = len(r)
+	}
+	spiTX(spi.Bus, wptr, wlen, rptr, rlen)
+	return nil
+}
+
 //export __tinygo_spi_configure
 func spiConfigure(bus uint8, sck Pin, SDO Pin, SDI Pin)
 
 //export __tinygo_spi_transfer
 func spiTransfer(bus uint8, w uint8) uint8
 
+//export __tinygo_spi_tx
+func spiTX(bus uint8, wptr *byte, wlen int, rptr *byte, rlen int) uint8
+
 // InitADC enables support for ADC peripherals.
 func InitADC() {
 	// Nothing to do here.

src/machine/spi_tx.go

@@ -1,4 +1,4 @@
-//go:build !baremetal || atmega || fe310 || k210 || (nxp && !mk66f18) || (stm32 && !stm32f7x2 && !stm32l5x2)
+//go:build atmega || fe310 || k210 || (nxp && !mk66f18) || (stm32 && !stm32f7x2 && !stm32l5x2)
 
 // This file implements the SPI Tx function for targets that don't have a custom
 // (faster) implementation for it.