LoongArch: Fix ICE for illegal strings in the target attribute.

LoongArch: Fix ICE for illegal strings in the target attribute.

Modify the two situations:

1. __attribute__ ((target ("arch")))
ICE will be reported before modification, and there will be an error
prompt after modification.

2. __attribute__ ((target ("arch=12345")))
Fixed the issue where the attribute string was not printed completely
in the previous error message.

gcc/ChangeLog:

	* config/loongarch/loongarch-target-attr.cc
	(loongarch_process_one_target_attr): Fix ICE.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/attr-check-error-message.c: Add tests.