[Darwin] Amend section for constants with reloactions.

Darwin's linker doesn't like text section relocations (they require special
enabling).  At least, the Fortran FE seems to generate cases where the
initialiser for a pointer constant can need a relocation.  We can handle
this by special-casing SECCAT_RODATA when the relocation is present by
placing the constant in the .const_data section.

gcc/ChangeLog:

2019-10-05  Iain Sandoe  <iain@sandoe.co.uk>

	PR target/59888
	* config/darwin.c (darwin_rodata_section): Add relocation flag,
	choose const_data section for constants with relocations.
	(machopic_select_section): Pass relocation flag to
	darwin_rodata_section ().

From-SVN: r276623

gcc/ChangeLog

+2019-10-05  Iain Sandoe  <iain@sandoe.co.uk>
+
+	PR target/59888
+	* config/darwin.c (darwin_rodata_section): Add relocation flag,
+	choose const_data section for constants with relocations.
+	(machopic_select_section): Pass relocation flag to
+	darwin_rodata_section ().
+
 2019-10-05  Jakub Jelinek  <jakub@redhat.com>
 
 	PR tree-optimization/91734

gcc/config/darwin.c

@@ -1251,12 +1251,13 @@ darwin_mark_decl_preserved (const char *name)
 }
 
 static section *
-darwin_rodata_section (int use_coal, bool zsize)
+darwin_rodata_section (int use_coal, bool zsize, int reloc)
 {
   return (use_coal
 	  ? darwin_sections[const_coal_section]
 	  : (zsize ? darwin_sections[zobj_const_section]
-		   : darwin_sections[const_section]));
+		   : reloc ? darwin_sections[const_data_section]
+			   : darwin_sections[const_section]));
 }
 
 static section *
@@ -1549,7 +1550,7 @@ machopic_select_section (tree decl,
 
     case SECCAT_RODATA:
     case SECCAT_SRODATA:
-      base_section = darwin_rodata_section (use_coal, zsize);
+      base_section = darwin_rodata_section (use_coal, zsize, reloc);
       break;
 
     case SECCAT_RODATA_MERGE_STR: