[sim/mips] Proposed change to mips_core_signal

[Ben Elliston]

I am working on a simulator port to a MIPS-like core that has a vector
processing unit.  To produce more helpful diagnostic messages, it
would useful if mips_core_signal() could differentiate between scalar
memory transfers and vector memory transfers.

Are there any objections to adding two new elements to the
_transfer_type enum in sim-basics.h to identify such transfers?

Ben