rts/linker: Only mprotect GOT after it is filled

This fixes a regression, introduced by 67c422ca, where we mprotect'd the
global offset table (GOT) region to PROT_READ before we had finished
filling it, resulting in a linker crash.

Fixes #16779.

rts/linker/elf_got.c

@@ -64,8 +64,6 @@ makeGot(ObjectCode * oc) {
                     symTab->symbols[i].got_addr
                             = (uint8_t *)oc->info->got_start
                               + (slot++ * sizeof(void*));
-        if(mprotect(mem, oc->info->got_size, PROT_READ) != 0) {
-            sysErrorBelch("unable to protect memory");
         }
     }
     return EXIT_SUCCESS;
@@ -115,6 +113,11 @@ fillGot(ObjectCode * oc) {
             }
         }
     }
+
+    // We are done initializing the GOT; freeze it.
+    if(mprotect(oc->info->got_start, oc->info->got_size, PROT_READ) != 0) {
+        sysErrorBelch("unable to protect memory");
+    }
     return EXIT_SUCCESS;
 }