gdb: remove SYMTAB_COMPUNIT macro, add getter/setter

Add a getter and a setter for a symtab's compunit_symtab.  Remove the
corresponding macro and adjust all callers.

For brevity, I chose the name "compunit" instead of "compunit_symtab"
the the field, getter and setter names.  Since we are already in symtab
context, the _symtab suffix seems redundant.

Change-Id: I4b9b731c96e3594f7733e75af1e3d01bc0e4fe92

gdb/breakpoint.c

@@ -9230,7 +9230,7 @@ resolve_sal_pc (struct symtab_and_line *sal)
       struct symbol *sym;
 
       bv = blockvector_for_pc_sect (sal->pc, 0, &b,
-				    SYMTAB_COMPUNIT (sal->symtab));
+				    sal->symtab->compunit ());
       if (bv != NULL)
 	{
 	  sym = block_linkage_function (b);