[rs6000] Fix a wrong GC issue

The source file rs6000.c was split up into several smaller source files
through commit 1acf0246.  However, variable "altivec_builtin_mask_for_load" and
"builtin_mode_to_type[MAX_MACHINE_MODE][2]" were marked with the wrong syntax
"GTY(([options])) type name", which led these two variables were not marked as
roots correctly and wrongly GCed.  And when "altivec_builtin_mask_for_load"
was wrongly GCed, the compiling for openJDK is failed with ICEs enabling
precompiled header under mcpu=power7.  So roots must be declared using one of
the following syntaxes: "extern GTY(([options])) type name;" and "static
GTY(([options])) type name;".

And the following patch adds variable "altivec_builtin_mask_for_load" and
"builtin_mode_to_type[MAX_MACHINE_MODE][2]" into the roots array.

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

gcc/ChangeLog

2020-03-11  Bin Bin Lv  <shlb@linux.ibm.com>

	* config/rs6000/rs6000-internal.h (altivec_builtin_mask_for_load,
	builtin_mode_to_type): Remove the declaration.
	* config/rs6000/rs6000.h (altivec_builtin_mask_for_load,
	builtin_mode_to_type): Add an extern GTY(()) declaration.
	* config/rs6000/rs6000.c (altivec_builtin_mask_for_load,
	builtin_mode_to_type): Remove the GTY(()) declaration.

gcc/config/rs6000/rs6000-internal.h

@@ -187,7 +187,5 @@ extern bool rs6000_passes_long_double;
 extern bool rs6000_passes_vector;
 extern bool rs6000_returns_struct;
 extern bool cpu_builtin_p;
-extern GTY(()) tree builtin_mode_to_type[MAX_MACHINE_MODE][2];
-extern GTY(()) tree altivec_builtin_mask_for_load;
 
 #endif

gcc/config/rs6000/rs6000.c

@@ -99,7 +99,7 @@
 #endif
 
 /* Support targetm.vectorize.builtin_mask_for_load.  */
-GTY(()) tree altivec_builtin_mask_for_load;
+tree altivec_builtin_mask_for_load;
 
 #ifdef USING_ELFOS_H
 /* Counter for labels which are to be placed in .fixup.  */
@@ -196,7 +196,7 @@ enum reg_class rs6000_constraints[RS6000_CONSTRAINT_MAX];
 int rs6000_vector_align[NUM_MACHINE_MODES];
 
 /* Map selected modes to types for builtins.  */
-GTY(()) tree builtin_mode_to_type[MAX_MACHINE_MODE][2];
+tree builtin_mode_to_type[MAX_MACHINE_MODE][2];
 
 /* What modes to automatically generate reciprocal divide estimate (fre) and
    reciprocal sqrt (frsqrte) for.  */

gcc/config/rs6000/rs6000.h

@@ -2490,6 +2490,9 @@ extern GTY(()) tree rs6000_builtin_types[RS6000_BTI_MAX];
 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;
+
 /* A C structure for machine-specific, per-function data.
    This is added to the cfun structure.  */
 typedef struct GTY(()) machine_function