Add a "fork" of musl as //fusl.

This is musl verbatim at d5f8394f6ea9549607567bd92de12a2446c15614.

See http://www.musl-libc.org/.

R=kulakowski@chromium.org

Review URL: https://codereview.chromium.org/1573973002 .
diff --git a/fusl/src/stdio/putc.c b/fusl/src/stdio/putc.c
new file mode 100644
index 0000000..fa89349
--- /dev/null
+++ b/fusl/src/stdio/putc.c
@@ -0,0 +1,12 @@
+#include "stdio_impl.h"
+
+int putc(int c, FILE *f)
+{
+	if (f->lock < 0 || !__lockfile(f))
+		return putc_unlocked(c, f);
+	c = putc_unlocked(c, f);
+	__unlockfile(f);
+	return c;
+}
+
+weak_alias(putc, _IO_putc);