Fix Invalid Operands being accepted for cm.lbu, cm.lhu, cm.lh, cm.sb, cm.sh

	* gas/config/tc-riscv.c: Fixed Invalid operands being accepted
	for cm.lbu, cm.lhu, cm.lh, cm.sb, and cm.sh. If uimm < 4, the
	instruction is illegal as the encoding is designated
	for custom use. Fixing this also Fixed an issue where instructions
	could be promoted to this illegal instructions (i.e. for sake of
	compression).
	* gas/testsuite/gas/riscv/zc-zcmb-fail-arch-conflict-0.s: Updated
	tests so that they no longer give false negatives for a
	relaxation issue.
	* gas/testsuite/gas/riscv/zc-zcmb-fail-arch-conflict-1.s: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-lb.d: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-lb.s: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-lbu.d: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-lbu.s: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-lh.d: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-lh.s: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-lhu.d: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-lhu.s: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-march-accept.d: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-march-accept.s: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-sb.d: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-sb.s: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-sh.d: Likewise.
	* gas/testsuite/gas/riscv/zc-zcmb-sh.s: Likewise.