arch/risc-v: Enable VPU initialization for QEMU RV

[ENJOU1224]

Summary

This PR fixes a runtime crash on QEMU rv-virt when the RISC-V Vector extension is enabled.

The riscv_vpuconfig() function, responsible for initializing the mstatus.VS bit (Vector Status), was defined but never called during the board-specific startup in qemu_rv_start() for the rv-virt platform. This oversight caused mstatus.VS to remain in the Off state (0x00), leading to an Illegal Instruction exception (MCAUSE: 0x2) when the OS attempted to save or restore vector context .

This patch adds the necessary call to riscv_vpuconfig() in arch/risc-v/src/qemu-rv/qemu_rv_start.c, ensuring the VPU is correctly initialized on boot.

Impact

• Is new feature added? No.
• Is existing feature changed? Yes. Fixes broken Vector extension support on QEMU.
• Impact on user: Users enabling RISC-V Vector extensions on QEMU (rv-virt) can now compile and boot successfully without runtime crashes. Users not enabling Vector extensions are unaffected.
• Impact on build: No direct impact on the build process.
• Impact on hardware: Validated on QEMU RISC-V 32-bit. No changes to physical hardware are implied.
• Impact on documentation: No.
• Impact on security: No.
• Impact on compatibility: This fix addresses a pre-existing runtime failure. It improves compatibility for systems configured with RISC-V Vector extensions.

Testing

Host Environment:

• OS: Ubuntu 25.10 (Linux 6.17.0-8-generic)
• Compiler: riscv64-unknown-elf-gcc (14.2.0+19) 14.2.0
• QEMU: QEMU emulator version 10.1.50 (v10.1.0-1314-gf3f2ad1193)

Verification Steps:

1. Build Verification:

   • Target: rv-virt:nsh (Configured with CONFIG_ARCH_RV_ISA_V=y).
   • Command: make -j
   • Result: Build completes successfully (provided that prerequisite type-fixing patch PR arch/risc-v: Fix incompatible pointer types in vector context switching #17762 is applied).

2. Runtime Verification:

   • Command:

     qemu-system-riscv32 \
           -nographic \
           -M virt \
           -bios none \
           -kernel ./nuttx \
           -cpu max,vlen=256,elen=64

   • Before check: System boots to NSH, but after executing a few NSH commands (typically within 2-3 commands) crashes with:

       riscv_exception: EXCEPTION: Illegal instruction. MCAUSE: 00000002, EPC: 80000xxx, MTVAL: 0c3073d7  # vsetvli instruction
   

   This occurs because mstatus.VS is not correctly initialized, preventing vector instructions from executing.

   • After check: System boots to NSH and runs stably, allowing execution of NSH commands (e.g., ls, ps). The Illegal Instruction exception is no longer observed.

   Boot Log (After Fix):

   ABC
   NuttShell (NSH) NuttX-12.12.0
   nsh> ls
   /:
    dev/
    proc/
   nsh> ps
     TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK    USED FILLED COMMAND
       0     0     0   0 FIFO     Kthread   - Ready              0000000000000000 0002032 0000420  20.6%  Idle_Task
       1     1     0 100 RR       Task      - Running            0000000000000000 0002000 0001636  81.8%! nsh_main
   nsh>