[AArch64] PR target/63596, honor tree-stdarg analysis result to improve VAARG codegen

gcc/
	PR target/63596
	* config/aarch64/aarch64.c (aarch64_expand_builtin_va_start): Honor
	tree-stdarg analysis results.
	(aarch64_setup_incoming_varargs): Likewise.

gcc/testsuite/
	* gcc.target/aarch64/va_arg_1.c: New testcase.
	* gcc.target/aarch64/va_arg_2.c: Likewise.
	* gcc.target/aarch64/va_arg_3.c: Likewise.

From-SVN: r236819