selftests/nolibc: fix test_file_stream() on musl libc

[ Upstream commit 8ba600a ]

fwrite() modifying errno is non-standard.

Only validate this behavior on those libc implementations which
implement it.

Fixes: a5f00be ("tools/nolibc: Add a simple test for writing to a FILE and reading it back")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: t-8ch

tools/testing/selftests/nolibc/nolibc-test.c

@@ -74,6 +74,14 @@ static const int is_nolibc =
 #endif
 ;
 
+static const int is_glibc =
+#ifdef __GLIBC__
+	1
+#else
+	0
+#endif
+;
+
 /* definition of a series of tests */
 struct test {
 	const char *name;              /* test name */
@@ -866,7 +874,7 @@ int test_file_stream(void)
 
 	errno = 0;
 	r = fwrite("foo", 1, 3, f);
-	if (r != 0 || errno != EBADF) {
+	if (r != 0 || ((is_nolibc || is_glibc) && errno != EBADF)) {
 		fclose(f);
 		return -1;
 	}