Compress loads/stores with implicit 0 offset.

Compress loads/stores with implicit 0 offset.

	gas/
	* config/tc-riscv.c (riscv_handle_implicit_zero_offset): New.
	(riscv_ip): Cases 'k', 'l', 'm', 'n', 'M', 'N', add call to
	riscv_handle_implicit_zero_offset.  At label load_store, replace
	existing code with call to riscv_handle_implicit_zero_offset.
	* testsuite/gas/riscv/c-ld.d, testsuite/gas/riscv/c-ld.s: New.
	* testsuite/gas/riscv/c-lw.d, testsuite/gas/riscv/c-lw.s: New.
	* testsuite/gas/riscv/riscv.exp: Run new tests.