cgo: add support for printf

The C printf function is sometimes needed for C files included using
CGo. This commit makes sure they're available on all systems where CGo
is fully supported (that is, everywhere except on AVR).

For baremetal systems using picolibc, I've picked the integer-only
version of printf to save on flash size. We might want to consider
providing a way to pick the floating point version instead, if needed.

testdata/cgo/main.c

@@ -1,5 +1,6 @@
 #include <math.h>
 #include "main.h"
+#include <stdio.h>
 
 int global = 3;
 bool globalBool = 1;
@@ -72,3 +73,7 @@ void arraydecay(int buf1[5], int buf2[3][8], int buf3[4][7][2]) {
 double doSqrt(double x) {
 	return sqrt(x);
 }
+
+void printf_single_int(char *format, int arg) {
+	printf(format, arg);
+}

testdata/cgo/main.go

@@ -179,6 +179,10 @@ func main() {
 	// libc: test basic stdio functionality
 	putsBuf := []byte("line written using C puts\x00")
 	C.puts((*C.char)(unsafe.Pointer(&putsBuf[0])))
+
+	// libc: test whether printf works in C.
+	printfBuf := []byte("line written using C printf with value=%d\n\x00")
+	C.printf_single_int((*C.char)(unsafe.Pointer(&printfBuf[0])), -21)
 }
 
 func printUnion(union C.joined_t) C.joined_t {

testdata/cgo/main.h

@@ -152,3 +152,5 @@ typedef int arraydecay_buf3[4][7][2];
 void arraydecay(int buf1[5], int buf2[3][8], arraydecay_buf3 buf3);
 
 double doSqrt(double);
+
+void printf_single_int(char *format, int arg);

testdata/cgo/out.txt

@@ -77,3 +77,4 @@ copied string: foobar
 CGo sqrt(3): +1.732051e+000
 C   sqrt(3): +1.732051e+000
 line written using C puts
+line written using C printf with value=-21