macro: Introduce UTF8() macro to define UTF-8 string literal

C23 changed char8_t from char to unsigned char, hence assigning a u8 literal
to const char* emits pointer sign warning, e.g.
========
../src/shared/qrcode-util.c: In function ‘print_border’:
../src/shared/qrcode-util.c:16:34: warning: pointer targets in passing argument 1 of ‘fputs’ differ in signedness [-Wpointer-sign]
   16 | #define UNICODE_FULL_BLOCK       u8"█"
      |                                  ^~~~~
      |                                  |
      |                                  const unsigned char *
../src/shared/qrcode-util.c:65:39: note: in expansion of macro ‘UNICODE_FULL_BLOCK’
   65 |                                 fputs(UNICODE_FULL_BLOCK, output);
      |                                       ^~~~~~~~~~~~~~~~~~
========

This introduces UTF8() macro, which define u8 literal and casts to consth char*,
then rewrites all u8 literal definitions with the macro.
With this change, we can build systemd with C23.

src/hostname/hostnamectl.c

@@ -70,21 +70,21 @@ typedef struct StatusInfo {
 
 static const char* chassis_string_to_glyph(const char *chassis) {
         if (streq_ptr(chassis, "laptop"))
-                return u8"💻"; /* Personal Computer */
+                return UTF8("💻"); /* Personal Computer */
         if (streq_ptr(chassis, "desktop"))
-                return u8"🖥️"; /* Desktop Computer */
+                return UTF8("🖥️"); /* Desktop Computer */
         if (streq_ptr(chassis, "server"))
-                return u8"🖳"; /* Old Personal Computer */
+                return UTF8("🖳"); /* Old Personal Computer */
         if (streq_ptr(chassis, "tablet"))
-                return u8"具"; /* Ideograph tool, implement; draw up, write, looks vaguely tabletty */
+                return UTF8("具"); /* Ideograph tool, implement; draw up, write, looks vaguely tabletty */
         if (streq_ptr(chassis, "watch"))
-                return u8"⌚"; /* Watch */
+                return UTF8("⌚"); /* Watch */
         if (streq_ptr(chassis, "handset"))
-                return u8"🕻"; /* Left Hand Telephone Receiver */
+                return UTF8("🕻"); /* Left Hand Telephone Receiver */
         if (streq_ptr(chassis, "vm"))
-                return u8"🖴"; /* Hard disk */
+                return UTF8("🖴"); /* Hard disk */
         if (streq_ptr(chassis, "container"))
-                return u8"☐"; /* Ballot Box  */
+                return UTF8("☐"); /* Ballot Box  */
         return NULL;
 }