RTEMS | risc-v: support running in s-mode (!1086)

[Gedare Bloom (@gedare)]

Gedare Bloom created a merge request: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1086

Project:Branches: gedare/rtems:riscv-smode to rtems/rtos/rtems:main
Author:   Gedare Bloom
Assignee: Gedare Bloom


## Summary

Updates #3337 

Adds a new CPU configuration option `RISCV_USE_S_MODE` that enables booting and running RTEMS using the supervisor privileged mode (s-mode) instead of the machine privileged mode (m-mode). This is implemented only for the generic `riscv` BSP variants and works only for uniprocessor execution with the `Sstc` extension enabled (for s-mode timer access). We need to port Supervisor Binary Interface (SBI) calls to access the m-mode legacy timer, IPIs to support SMP, and console input.

It can be run using 
`qemu-system-riscv64 -M virt -nographic -kernel build/riscv/rv64imafdc_s/testsuites/samples/hello.exe`

with config.ini:
```
[riscv/rv64imafdc_s]
INHERIT = rv64imafdc
RISCV_RAM_REGION_BEGIN = 0x84000000
RISCV_RAM_REGION_SIZE =  0x03000000
RISCV_USE_S_MODE = True
```

There are about mixed failures whether using s-mode or m-mode.

Test results with S-Mode:
```
Passed:        644
Failed:         17
User Input:      6
Expected Fail:  24
Indeterminate:   0
Benchmark:       3
Timeout:         6
Test too long:   0
Invalid:         0
Wrong Version:   0
Wrong Build:     0
Wrong Tools:     0
Wrong Header:    0
------------------
Total:         700

Failures:
 dl01.exe
 dl02.exe
 dl05.exe
 dl06.exe
 dl07.exe
 dl08.exe
 dl09.exe
 dl11.exe
 dl12.exe
 fsfatfsformat01.exe
 psx09.exe
 psxstat.exe
 sp69.exe
 stringto01.exe
 tftpfs.exe
 ts-validation-one-cpu-0.exe
 ttest01.exe
...
Timeouts:
 spconsole01.exe
 tm27.exe
 ts-performance-no-clock-0.exe
 ts-unit-no-clock-0.exe
 ts-validation-intr.exe
 ts-validation-no-clock-0.exe
```

Test results without S-Mode:
```
Passed:        648
Failed:         19
User Input:      6
Expected Fail:  24
Indeterminate:   0
Benchmark:       3
Timeout:         0
Test too long:   0
Invalid:         0
Wrong Version:   0
Wrong Build:     0
Wrong Tools:     0
Wrong Header:    0
------------------
Total:         700

Failures:
 dl01.exe
 dl02.exe
 dl05.exe
 dl06.exe
 dl07.exe
 dl08.exe
 dl09.exe
 dl11.exe
 dl12.exe
 fsfatfsformat01.exe
 psx09.exe
 psx12.exe
 psxstat.exe
 sp69.exe
 spcpucounter01.exe
 stringto01.exe
 tftpfs.exe
 ts-validation-intr.exe
 ttest01.exe
```

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