esp32: enable the FPU

This allows working with float32 values, for example it allows
testdata/float.go to work correctly (assuming an Xtensa backend bug is
fixed, see https://github.com/espressif/llvm-project/issues/41).

src/device/esp/esp32.S

@@ -42,6 +42,11 @@ call_start_cpu0:
     wsr.ps a2
     rsync
 
+    // Enable the FPU (coprocessor 0 so the lowest bit).
+    movi a2, 1
+    wsr.cpenable a2
+    rsync
+
     // Jump to the runtime start function written in Go.
     call4 main