Disable rv32ua/rv64ua LR/SC test case 4 (#316)

- After discussion in riscv/riscv-tests#315, disable this test case, as it
  makes implementation assumptions which are not valid with respect to the
  specification.

- Leave code present but commented out.

 On branch dev/benm-disable-lrsc-test-4
 Changes to be committed:
	modified:   isa/rv64ua/lrsc.S