Optimize sscanf

Issue:I7NQ5W
Signed-off-by: Nyinchuang <yinchuang@huawei.com>
Test:libctest

musl_src.gni

@@ -2190,6 +2190,7 @@ musl_src_porting_file = [
   "src/stdio/stderr.c",
   "src/stdio/fgets.c",
   "src/stdio/vsnprintf.c",
+  "src/stdio/vsscanf.c",
   "src/internal/stdio_impl.h",
   "src/internal/vdso.c",
   "src/time/clock_gettime.c",

porting/linux/user/src/stdio/vsscanf.c

+#include "stdio_impl.h"
+#include <string.h>
+
+// Empty implementation because vfscanf operates rpos directly.
+static size_t string_read(FILE *f, unsigned char *buf, size_t len)
+{
+	return 0;
+}
+
+int vsscanf(const char *restrict s, const char *restrict fmt, va_list ap)
+{
+	size_t s_len = strlen(s);
+	FILE f = {
+		.buf = (void *)s, .cookie = (void *)s,
+		.read = string_read, .lock = -1,
+		.buf_size = s_len, .rpos = s, .rend = s + s_len,
+	};
+	return vfscanf(&f, fmt, ap);
+}
+
+weak_alias(vsscanf,__isoc99_vsscanf);