[NEW DYNAREC ARM] Use ARM mode explicitly - some systems use -mthumb by ...

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Some gnu/linux arm systems (e.g. debian armhf) use --with-mode=thumb instead with --with-mode=arm (gcc runtime -mthumb/-marm options), breaking the new arm dynarec.
This fixes compilation on these systems.

See https://github.com/Gillou68310/mupen64plus-ae/issues/3