tests/tcg: target/mips: Add tests for MIPS64R6 int multiply instructions

Add tests for MIPS64R6 integer multiply instructions: MUL, MUH, MULU,
MUHU, DMUL, DMUH, DMULU, and DMUHU.

MUH and MUHU require 64 bit inputs in the form of 64-bit sign-extended
32-bit inputs.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1551800076-8104-14-git-send-email-aleksandar.markovic@rt-rk.com>