LoongArch: Fixed a compilation failure with '%c' in inline assembly [PR107731].

LoongArch: Fixed a compilation failure with '%c' in inline assembly [PR107731].

Co-authored-by: Yang Yujie <yangyujie@loongson.cn>
	PR target/107731

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_classify_address):
	Add precessint for CONST_INT.
	(loongarch_print_operand_reloc): Operand modifier 'c' is supported.
	(loongarch_print_operand): Increase the processing of '%c'.
	* doc/extend.texi: Adds documents for LoongArch operand modifiers.
	And port the public operand modifiers information to this document.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/tst-asm-const.c: Moved to...
	* gcc.target/loongarch/pr107731.c: ...here.