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.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);