[RS6000] Correct PIC_OFFSET_TABLE_REGNUM

Leaving this as r30 results in pic_offset_table_rtx of (reg 30)
for -m64, which is completely bogus.  Various rtl analysis predicate
functions treat pic_offset_table_rtx specially..

	* config/rs6000/rs6000.h (PIC_OFFSET_TABLE_REGNUM): Correct.

From-SVN: r235876

gcc/ChangeLog

 2016-05-04  Alan Modra  <amodra@gmail.com>
 
+	* config/rs6000/rs6000.h (PIC_OFFSET_TABLE_REGNUM): Correct.
 	* config/rs6000/sysv4.h (TARGET_TOC): Simplify.
 	* config/rs6000/rs6000.c (rs6000_elf_output_toc_section_asm_op):
 	Align .toc.

gcc/config/rs6000/rs6000.h

@@ -2058,7 +2058,10 @@ do {									     \
    to allocate such a register (if necessary).  */
 
 #define RS6000_PIC_OFFSET_TABLE_REGNUM 30
-#define PIC_OFFSET_TABLE_REGNUM (flag_pic ? RS6000_PIC_OFFSET_TABLE_REGNUM : INVALID_REGNUM)
+#define PIC_OFFSET_TABLE_REGNUM \
+  (TARGET_TOC ? TOC_REGISTER			\
+   : flag_pic ? RS6000_PIC_OFFSET_TABLE_REGNUM	\
+   : INVALID_REGNUM)
 
 #define TOC_REGISTER (TARGET_MINIMAL_TOC ? RS6000_PIC_OFFSET_TABLE_REGNUM : 2)