riscv64: spec compliance and regression test fix

[akifejaz]

This PR corrects the RISC-V64 GNU port as per spec and its QEMU virt support.

• Fix RISC-V64 GNU port and QEMU virt BSP for spec compliance and proper FP/vector context saving.
• Enforce atomic interrupt state operations, correct stack frame layouts, and add compile-time ABI guards.
• Refactor PLIC and CLINT drivers with volatile memory accesses, proper indexing, and exception skip logic.
• Update linker scripts, global pointer initialization, and CMake functional test targets.

Regression 475/475 on QEMU 6.2 (5 configs x 95), compile matrix for lp64d, lp64f, lp64 and rv64gcv, ecall/ebreak skip checked on RV64 and RV32.