AArch64: Cleanup aarch64_classify_symbol

Use a GOT indirection for extern weak symbols instead of a literal - this is
the same as PIC/PIE and mirrors LLVM behaviour.  Ensure PIC/PIE use the same
offset limits for symbols that don't use the GOT.

Passes bootstrap and regress.

ChangeLog:
2021-04-27  Wilco Dijkstra  <wdijkstr@arm.com>

	* config/aarch64/aarch64.c (aarch64_classify_symbol): Use GOT for
	extern weak symbols.  Limit symbol offsets for non-GOT symbols with
	PIC/PIE.

Author: Wilco Dijkstra

gcc/config/aarch64/aarch64.c

@@ -17902,50 +17902,45 @@ aarch64_classify_symbol (rtx x, HOST_WIDE_INT offset)
 
       switch (aarch64_cmodel)
 	{
+	case AARCH64_CMODEL_TINY_PIC:
 	case AARCH64_CMODEL_TINY:
+	  /* With -fPIC non-local symbols use the GOT.  For orthogonality
+	     always use the GOT for extern weak symbols.  */
+	  if ((flag_pic || SYMBOL_REF_WEAK (x))
+	      && !aarch64_symbol_binds_local_p (x))
+	    return SYMBOL_TINY_GOT;
+
 	  /* When we retrieve symbol + offset address, we have to make sure
 	     the offset does not cause overflow of the final address.  But
 	     we have no way of knowing the address of symbol at compile time
 	     so we can't accurately say if the distance between the PC and
 	     symbol + offset is outside the addressible range of +/-1MB in the
 	     TINY code model.  So we limit the maximum offset to +/-64KB and
 	     assume the offset to the symbol is not larger than +/-(1MB - 64KB).
-	     If offset_within_block_p is true we allow larger offsets.
-	     Furthermore force to memory if the symbol is a weak reference to
-	     something that doesn't resolve to a symbol in this module.  */
-
-	  if (SYMBOL_REF_WEAK (x) && !aarch64_symbol_binds_local_p (x))
-	    return SYMBOL_FORCE_TO_MEM;
+	     If offset_within_block_p is true we allow larger offsets.  */
 	  if (!(IN_RANGE (offset, -0x10000, 0x10000)
 		|| offset_within_block_p (x, offset)))
 	    return SYMBOL_FORCE_TO_MEM;
 
 	  return SYMBOL_TINY_ABSOLUTE;
 
+
+	case AARCH64_CMODEL_SMALL_SPIC:
+	case AARCH64_CMODEL_SMALL_PIC:
 	case AARCH64_CMODEL_SMALL:
+	  if ((flag_pic || SYMBOL_REF_WEAK (x))
+	      && !aarch64_symbol_binds_local_p (x))
+	    return aarch64_cmodel == AARCH64_CMODEL_SMALL_SPIC
+		    ? SYMBOL_SMALL_GOT_28K : SYMBOL_SMALL_GOT_4G;
+
 	  /* Same reasoning as the tiny code model, but the offset cap here is
 	     1MB, allowing +/-3.9GB for the offset to the symbol.  */
-
-	  if (SYMBOL_REF_WEAK (x) && !aarch64_symbol_binds_local_p (x))
-	    return SYMBOL_FORCE_TO_MEM;
 	  if (!(IN_RANGE (offset, -0x100000, 0x100000)
 		|| offset_within_block_p (x, offset)))
 	    return SYMBOL_FORCE_TO_MEM;
 
 	  return SYMBOL_SMALL_ABSOLUTE;
 
-	case AARCH64_CMODEL_TINY_PIC:
-	  if (!aarch64_symbol_binds_local_p (x))
-	    return SYMBOL_TINY_GOT;
-	  return SYMBOL_TINY_ABSOLUTE;
-
-	case AARCH64_CMODEL_SMALL_SPIC:
-	case AARCH64_CMODEL_SMALL_PIC:
-	  if (!aarch64_symbol_binds_local_p (x))
-	    return (aarch64_cmodel == AARCH64_CMODEL_SMALL_SPIC
-		    ?  SYMBOL_SMALL_GOT_28K : SYMBOL_SMALL_GOT_4G);
-	  return SYMBOL_SMALL_ABSOLUTE;
-
 	case AARCH64_CMODEL_LARGE:
 	  /* This is alright even in PIC code as the constant
 	     pool reference is always PC relative and within