LoongArch: Define macro CLEAR_INSN_CACHE.

LoongArch: Define macro CLEAR_INSN_CACHE.

LoongArch's microstructure ensures cache consistency by hardware.
Due to out-of-order execution, "ibar" is required to ensure the visibility of the
store (invalidated icache) executed by this CPU before "ibar" (to the instance).
"ibar" will not invalidate the icache, so the start and end parameters are not Affect
"ibar" performance.

gcc/ChangeLog:

	* config/loongarch/loongarch.h (CLEAR_INSN_CACHE): New definition.

(cherry picked from commit 5697ed0327f23d2e2ec4f7beec3b3d02f463173c)