tests/tcg: target/mips: Fix test utilities for 128-bit tests

Add "static" and "const" modifiers where appropriate, and fix other
minor issues.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1551800076-8104-7-git-send-email-aleksandar.markovic@rt-rk.com>