parallel compute common block

src/linker/lnk.h

@@ -89,6 +89,11 @@ typedef struct
     struct {
       HashTable **defns;
     } gather_sects;
+    struct {
+      U64                    *counts;
+      U64                    *offsets;
+      LNK_CommonBlockContrib *contribs;
+    } common_block;
     struct {
       LNK_SectionContribChunk **chunks;
     } sort_contribs;