torture: Add init-program support for loongarch

This commit adds the __loongarch__, __loongarch_lp64, and
__loongarch_double_float targets to rcutorture's mkinitrd.sh
script in order to allow nolibc init programs for loongarch.

[ paulmck: Apply feedback from Feiyang Chen. ]

Cc: Feiyang Chen <chenfeiyang@loongson.cn>
Cc: Huacai Chen <chenhuacai@loongson.cn>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>