cgo: add support for stdio in picolibc

This is simple: just add the required header file. But this commit also
adds a simple test to make sure it remains supported.

compileopts/config.go

@@ -206,7 +206,12 @@ func (c *Config) CFlags() []string {
 	}
 	if c.Target.Libc == "picolibc" {
 		root := goenv.Get("TINYGOROOT")
-		cflags = append(cflags, "-nostdlibinc", "-Xclang", "-internal-isystem", "-Xclang", filepath.Join(root, "lib", "picolibc", "newlib", "libc", "include"))
+		picolibcDir := filepath.Join(root, "lib", "picolibc", "newlib", "libc")
+		cflags = append(cflags,
+			"-nostdlibinc",
+			"-Xclang", "-internal-isystem", "-Xclang", filepath.Join(picolibcDir, "include"),
+			"-Xclang", "-internal-isystem", "-Xclang", filepath.Join(picolibcDir, "tinystdio"),
+		)
 		cflags = append(cflags, "-I"+filepath.Join(root, "lib/picolibc-include"))
 	}
 	// Always emit debug information. It is optionally stripped at link time.

testdata/cgo/main.go

@@ -1,6 +1,7 @@
 package main
 
 /*
+#include <stdio.h>
 int fortytwo(void);
 #include "main.h"
 int mul(int, int);
@@ -129,6 +130,9 @@ func main() {
 	buf2 := make([]byte, len(buf1))
 	C.strcpy((*C.char)(unsafe.Pointer(&buf2[0])), (*C.char)(unsafe.Pointer(&buf1[0])))
 	println("copied string:", string(buf2[:C.strlen((*C.char)(unsafe.Pointer(&buf2[0])))]))
+
+	// libc: test basic stdio functionality
+	println("SEEK_CUR:", C.SEEK_CUR)
 }
 
 func printUnion(union C.joined_t) C.joined_t {

testdata/cgo/out.txt

@@ -61,3 +61,4 @@ option 3A: 21
 enum width matches: true
 CFLAGS value: 17
 copied string: foobar
+SEEK_CUR: 1