Support handling gemmini exceptions through gemmini instruction return values

[jerryz123]

All gemmini instruction (except for flush) now have a return value, which indicates the status of gemmini at the time the instruction is issued. This allows the software to support handling gemmini exceptions at the user-level.

If wdata == 0, everything is going fine, there are no pending exceptions.
If wdata != 0, gemmini has a pending exception. While gemmini has a pending instructions, any issued instruction to gemmini returns wdata, without actually issuing into the accelerator. LSB of wdata indicates type of the exception, while the rest of the bits indicates the bad address.

Upon issuing any gemmini instruction, check wdata for the error case. If error, jump into the gemmini exception "handler", handle the exception, then attempt to reissue the instruction which just reported an error. This provides the illusion of precise exceptions. gemmini_fence now also checks for pending exceptions, and handles them.

These changes should have no effect on software which is compiled without the flag GEMMINI_XCPT_DEBUG, as that flag disables all checks of the status value. Software with GEMMINI_XCPT_DEBUG is tolerant to page faults, but slower, and as such is meant to be used to debug page faults on gemmini.

[jerryz123]

This appears to be a larger change than reality because my editor removed trailing whitespace from gemmini.h.