Add non-macro stdin/stdout/stderr too.

Various C and C++ standards explicitly say that stdin/stdout/stderr
should be macros, but glibc makes them global variables too. This
means it's possible to write code that uses those names as locals,
but that code (toybox being an example) won't build on bionic.

If we'd done this earlier, we could have hidden __sF for LP64, but
it's too late now.

Change-Id: I90cf8c73f52b66e1760b8fa2e135b9f9f9651230

libc/include/stdio.h

@@ -146,7 +146,16 @@ typedef	struct __sFILE {
 	fpos_t	_offset;	/* current lseek offset */
 } FILE;
 
+/* Legacy BSD implementation of stdin/stdout/stderr. */
 extern FILE __sF[];
+/* More obvious implementation. */
+extern FILE* stdin;
+extern FILE* stdout;
+extern FILE* stderr;
+/* C99 and earlier plus current C++ standards say these must be macros. */
+#define stdin stdin
+#define stdout stdout
+#define stderr stderr
 
 #define	__SLBF	0x0001		/* line buffered */
 #define	__SNBF	0x0002		/* unbuffered */
@@ -209,10 +218,6 @@ extern FILE __sF[];
 #define	SEEK_END	2	/* set file offset to EOF plus offset */
 #endif
 
-#define	stdin	(&__sF[0])
-#define	stdout	(&__sF[1])
-#define	stderr	(&__sF[2])
-
 /*
  * Functions defined in ANSI C standard.
  */