LoongArch: testsuite:Fixed a bug that added a target check error.

After the code is committed in r14-6948, GCC regression testing on some
architectures will produce the following error:

"error executing dg-final: unknown effective target keyword `loongarch*-*-*'"

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp: Removed an issue with "target keyword"
	checking errors on LoongArch architecture.