machine/stm32: add OTG FS USB driver for F4/F7

STM32F4 and F7 share the same OTG FS IP but have no USB driver in
TinyGo. This adds a full device-mode driver covering CDC, HID, and
MSC with working examples.

- OTG FS has 4 physical EPs (0–3); virtual indices 4–7 fold onto
  them via physEP() so the existing machine/usb API is unchanged
- F4 bypasses VBUS sensing via GCCFG.NOVBUSSENS; F7 uses the USB
  voltage regulator + GOTGCTL B-valid override instead
- STM32F7 PLL_Q changed 2→9 to produce the 48 MHz clock required
  by USB/RNG/SDMMC; CK48MSEL cleared to select main PLL as source
- HID and MSC descriptors remapped at init() to physical endpoint
  addresses (EP2/EP1 for HID, EP2/EP3 for MSC)
- usb-storage example replaced machine.Flash with a FAT12 RAM disk
  so the host mounts without reformatting
- MSC sendCSW sets queuedBytes before state transition to fix a
  missed byte-count on the status phase

builder/sizes_test.go

@@ -44,7 +44,7 @@ func TestBinarySize(t *testing.T) {
 		// microcontrollers
 		{"hifive1b", "examples/echo", 3771, 309, 0, 2260},
 		{"microbit", "examples/serial", 2832, 368, 8, 2256},
-		{"wioterminal", "examples/pininterrupt", 8053, 1663, 132, 7488},
+		{"wioterminal", "examples/pininterrupt", 8065, 1663, 132, 7488},
 
 		// TODO: also check wasm. Right now this is difficult, because
 		// wasm binaries are run through wasm-opt and therefore the