std.os.linux: Define the Stat struct for riscv32.

The kernel does define the struct, it just doesn't use it. Yet both glibc and
musl expose it directly as their public stat struct, and std.c takes it from
std.os.linux. So just define it after all.

lib/std/os/linux/riscv32.zig

@@ -212,8 +212,37 @@ pub const msghdr_const = extern struct {
     flags: i32,
 };
 
-/// No `Stat` structure on this platform, only `Statx`.
-pub const Stat = void;
+// The `stat` definition used by the Linux kernel.
+pub const Stat = extern struct {
+    dev: dev_t,
+    ino: ino_t,
+    mode: mode_t,
+    nlink: nlink_t,
+    uid: uid_t,
+    gid: gid_t,
+    rdev: dev_t,
+    __pad: usize,
+    size: off_t,
+    blksize: blksize_t,
+    __pad2: i32,
+    blocks: blkcnt_t,
+    atim: timespec,
+    mtim: timespec,
+    ctim: timespec,
+    __unused: [2]u32,
+
+    pub fn atime(self: @This()) timespec {
+        return self.atim;
+    }
+
+    pub fn mtime(self: @This()) timespec {
+        return self.mtim;
+    }
+
+    pub fn ctime(self: @This()) timespec {
+        return self.ctim;
+    }
+};
 
 pub const Elf_Symndx = u32;