std.c.*: add birthtime function to Stat

Adds a birthtime function to the `Stat` structs of Unices which support this. This is done to match the `atime`, `mtime`, and `ctime` functions.

lib/std/c/darwin.zig

@@ -395,6 +395,10 @@ pub const Stat = extern struct {
     pub fn ctime(self: @This()) timespec {
         return self.ctimespec;
     }
+
+    pub fn birthtime(self: @This()) timespec {
+        return self.birthtimespec;
+    }
 };
 
 pub const timespec = extern struct {

lib/std/c/freebsd.zig

@@ -283,6 +283,10 @@ pub const Stat = extern struct {
     pub fn ctime(self: @This()) timespec {
         return self.ctim;
     }
+
+    pub fn birthtime(self: @This()) timespec {
+        return self.birthtim;
+    }
 };
 
 pub const timespec = extern struct {

lib/std/c/netbsd.zig

@@ -312,6 +312,10 @@ pub const Stat = extern struct {
     pub fn ctime(self: @This()) timespec {
         return self.ctim;
     }
+
+    pub fn birthtime(self: @This()) timespec {
+        return self.birthtim;
+    }
 };
 
 pub const timespec = extern struct {

lib/std/c/openbsd.zig

@@ -235,6 +235,10 @@ pub const Stat = extern struct {
     pub fn ctime(self: @This()) timespec {
         return self.ctim;
     }
+
+    pub fn birthtime(self: @This()) timespec {
+        return self.birthtim;
+    }
 };
 
 pub const timespec = extern struct {