Use | instead of + for entrypgdir. Linker doesn't get in our way here.

main.c

@@ -106,9 +106,9 @@ startothers(void)
 __attribute__((__aligned__(PGSIZE)))
 pde_t entrypgdir[NPDENTRIES] = {
   // Map VA's [0, 4MB) to PA's [0, 4MB)
-  [0] = (0) + PTE_P + PTE_W + PTE_PS,
+  [0] = (0) | PTE_P | PTE_W | PTE_PS,
   // Map VA's [KERNBASE, KERNBASE+4MB) to PA's [0, 4MB)
-  [KERNBASE>>PDXSHIFT] = (0) + PTE_P + PTE_W + PTE_PS,
+  [KERNBASE>>PDXSHIFT] = (0) | PTE_P | PTE_W | PTE_PS,
 };
 
 //PAGEBREAK!