Add QEMU based CI regression test infra for RV32 and RV64 (#526)

Added a QEMU virt-machine BSP and CTest infrastructure to run the
ThreadX regression suite on both RISC-V 32-bit and 64-bit targets
in CI.

New components:
- BSP (entry, trap, PLIC, CLINT timer, UART, linker script) targeting
  QEMU virt machine for RV32 and RV64
- CMake build system with Ninja, supporting multiple build configs
- CI scripts: install_riscv.sh (toolchain + QEMU), build_tx_riscv.sh,
  test_tx_riscv.sh
- GitHub Actions workflow job for RISC-V regression gating

Port fixes:
- RV32 tx_thread_context_restore.S: set MPIE alongside MPP (0x1800 →
  0x1880) so mret re-enables interrupts
- RV32/RV64 tx_port.h: add TX_REGRESSION_TEST extension macros needed
  by the test harness
- RV32/RV64 example_build scripts: add compile and QEMU launch steps

Regression test portability fixes:
- Block memory tests: increase pool sizes (320 → 340) to accommodate
  larger RISC-V block-header alignment
- Byte memory test: replace hardcoded offsets with BYTE_POOL_OVERHEAD
  macro for portable pool-size computation
- Event flag timeout test: make counter tolerance unconditional,
  removing linux-only guard

Signed-off-by: Akif Ejaz <[email protected]>

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