[rtl] Add support for direct mode in mtvec

[leesum1]

The Ibex RISC-V core currently lacks support for the direct mode in the mtvec (#1419). This commit make mtvec support both vectored and direct mode.

To ensure compatibility with existing test programs, the initial value of mtvec.MODE is set to vectored mode.For both vectored mode and direct mode, mtvec.BASE must be 256-byte aligned.

Test

Pass test in rt-thread(direct mode)
Pass examples/sw/simple_system/hello_test (vectored mode)

[GregAC]

Thanks for your contribution @leesum1. The RTL change looks good but before we can merge this we need to add DV. We need to add tests to the DV environment with appropriate coverage. I'll leave this open til that's done. I've created an issue to track the DV needs #2047

[medexs]

Hi @leesum1 , @GregAC , what is the current status on the direct mode support for mtvec? From the checks above I can see that almost all of them but three passed successfully. When could be the support finished?

I am working on porting NuttX to Ibex and the support for the direct mode would be much appreciated as handling of all exceptions/interrupts in NuttX is done in only one routine.

[leesum1]

Hi @leesum1 , @GregAC , what is the current status on the direct mode support for mtvec? From the checks above I can see that almost all of them but three passed successfully. When could be the support finished?

I am working on porting NuttX to Ibex and the support for the direct mode would be much appreciated as handling of all exceptions/interrupts in NuttX is done in only one routine.

NO DV NOW，I don't know how to add the DV properly