Loongarch: Fix plugin header missing install.

gcc/ChangeLog:

	* config/loongarch/t-loongarch: Add loongarch-driver.h into
	TM_H. Add loongarch-def.h and loongarch-tune.h into
	OPTIONS_H_EXTRA.

Co-authored-by: Lulu Cheng <chenglulu@loongson.cn>
(cherry picked from commit 3e315736)