add UTIME_NOW and UTIME_OMIT constants for use in utimensat/futimens

copied from lib/libc/include/generic-musl/sys/stat.h

lib/std/os/bits/linux.zig

@@ -775,6 +775,9 @@ pub fn S_ISSOCK(m: u32) bool {
     return m & S_IFMT == S_IFSOCK;
 }
 
+pub const UTIME_NOW = 0x3fffffff;
+pub const UTIME_OMIT = 0x3ffffffe;
+
 pub const TFD_NONBLOCK = O_NONBLOCK;
 pub const TFD_CLOEXEC = O_CLOEXEC;