[rs6000] Fix a wrong GC issue

[rs6000] Fix a wrong GC issue

The source file rs6000.c was split up into several smaller source files
through commit 1acf024.  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.