target/mips: tests/tcg: Fix target configurations for MSA tests

At this moment, the only MIPS CPUs that are emulated in QEMU and
support MSA extension are R5600 (mips32r5), and I6400/I6500 (mips64r6).
Therefore, mips32r5 and mips64r6 are the only ISAs that could support
MSA in QEMU. This means mips32r6 currently do not make much sense, and
mips32r5 support for MSA tests is needed, which is done by this patch.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1566216496-17375-38-git-send-email-aleksandar.markovic@rt-rk.com>