cgo: add C._Bool type

This fixes https://github.com/tinygo-org/tinygo/issues/3926.

While working on this I've found another bug: if C.bool is referenced
from within Go, it isn't available anymore on the C side. This is an
existing bug that also applies to float and double, but may be less
likely to be triggered there.
This bug is something to be fixed at a later time (it has something to
do with preprocessor defines).

testdata/cgo/main.go

@@ -9,6 +9,7 @@ int mul(int, int);
 #include <string.h>
 #cgo CFLAGS: -DSOME_CONSTANT=17
 #define someDefine -5 + 2 * 7
+bool someBool;
 */
 import "C"
 
@@ -56,6 +57,9 @@ func main() {
 	var goInt8 int8 = 5
 	var _ C.int8_t = goInt8
 
+	var _ bool = C.someBool
+	var _ C._Bool = C.someBool
+
 	// more globals
 	println("bool:", C.globalBool, C.globalBool2 == true)
 	println("float:", C.globalFloat)