LoongArch: Fix issue where data marked as GTY is cleaned up by ggc.

As for GGC(GCC Garbage Collection), it's use gengtype tool to scan all
source files contain the GTY mark, and generate gt-*.h files. GGC
traversal these file to find gt_root node and marks these objects that
directly or indirectly reference this node as active, then clean up
unmarked object's memory.

For the loongarch-builtins.cc file, it is necessary to add
target_gtfiles in config.gcc to generate gt-loongarch-builtins.h, and
include this header file in the .cc file, prevented the data marked
with GTY in this `.cc` file cleaned up by ggc.

gcc/ChangeLog:

	* config.gcc: Add target_gtfiles.
	* config/loongarch/loongarch-builtins.cc: Add header file.

Author: zhaozhou

gcc/config.gcc

@@ -502,6 +502,7 @@ loongarch*-*-*)
 	extra_headers="larchintrin.h lsxintrin.h lasxintrin.h"
 	extra_objs="loongarch-c.o loongarch-builtins.o loongarch-cpu.o loongarch-opts.o loongarch-def.o loongarch-evolution.o loongarch-target-attr.o"
 	extra_gcc_objs="loongarch-driver.o loongarch-cpu.o loongarch-opts.o loongarch-def.o"
+	target_gtfiles="\$(srcdir)/config/loongarch/loongarch-builtins.cc"
 	extra_options="${extra_options} g.opt fused-madd.opt"
 	;;
 nds32*)

gcc/config/loongarch/loongarch-builtins.cc

@@ -3259,3 +3259,5 @@ loongarch_build_builtin_va_list (void)
 {
   return ptr_type_node;
 }
+
+#include "gt-loongarch-builtins.h"