[rs6000] Rewrite the declaration of a variable

Rewrite the declaration of toc_section from the source file rs6000.c to its
header file for standardizing the code.

Bootstrap and regression were done on powerpc64le-linux-gnu (LE) with no
regressions.

gcc/ChangeLog

2020-03-20  Bin Bin Lv  <shlb.linux.ibm.com>

	* config/rs6000/rs6000-internal.h (toc_section): Remove the
	declaration.
	* config/rs6000/rs6000.h (toc_section): Add the declaration.
	* config/rs6000/rs6000.c (toc_section): Remove the declaration.

gcc/config/rs6000/rs6000-internal.h

@@ -64,7 +64,6 @@ typedef struct rs6000_stack {
 extern int need_toc_init;
 extern char toc_label_name[10];
 extern int rs6000_pic_labelno;
-extern section *toc_section;
 
 #ifdef USING_ELFOS_H
 extern const char *rs6000_machine;

gcc/config/rs6000/rs6000.c

@@ -181,7 +181,6 @@ static GTY(()) section *tls_private_data_section;
 static GTY(()) section *read_only_private_data_section;
 static GTY(()) section *sdata2_section;
 
-extern GTY(()) section *toc_section;
 section *toc_section = 0;
 
 /* Describe the vector unit used for modes.  */

gcc/config/rs6000/rs6000.h

@@ -2492,6 +2492,7 @@ extern GTY(()) tree rs6000_builtin_decls[RS6000_BUILTIN_COUNT];
 #ifndef USED_FOR_TARGET
 extern GTY(()) tree builtin_mode_to_type[MAX_MACHINE_MODE][2];
 extern GTY(()) tree altivec_builtin_mask_for_load;
+extern GTY(()) section *toc_section;
 
 /* A C structure for machine-specific, per-function data.
    This is added to the cfun structure.  */