tests/tcg: target/mips: Add test utilities for 64-bit tests

Add test utilities for 64-bit tests. Some of MIPS64R6 instructions
require 64-bit inputs to be 32-bit integers sign-extedned to 64 bits,
hence the need for sets of such inputs.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1551800076-8104-6-git-send-email-aleksandar.markovic@rt-rk.com>