x86-64: Don't use temp for argument in a TImode register

Don't use temp for a PARALLEL BLKmode argument of an EXPR_LIST expression
in a TImode register.  Otherwise, the TImode variable will be put in
the GPR save area which guarantees only 8-byte alignment.

gcc/

	PR target/116621
	* config/i386/i386.cc (ix86_gimplify_va_arg): Don't use temp for
	a PARALLEL BLKmode container of an EXPR_LIST expression in a
	TImode register.

gcc/testsuite/

	PR target/116621
	* gcc.target/i386/pr116621.c: New test.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>