[bsp][qemu-virt64-riscv] Allow optional C-only Picolibc builds - #11849

拉取/合并请求描述：(PR description)

为什么提交这份PR (why to submit this PR)

The QEMU RV64 BSP unconditionally links libsupc++, even for a C-only
configuration. This prevents using a C-only GNU/Picolibc toolchain. Selecting
an external Picolibc specs file also currently requires editing rtconfig.py.

你的解决方案是什么 (what is your solution)

• Accept an optional RTT_PICOLIBC_SPECS environment variable and pass it to
  compilation, assembly and linking. Unset/empty leaves libc selection unchanged.
• Remove -lsupc++ only when RT_USING_CPLUSPLUS is disabled. C++ configurations
  retain the original linker flags and existing G++ driver selection.

No libc sources, default configuration, ISA/ABI, or other BSPs change.
AI assistance was used to prepare and validate this patch.

请提供验证的bsp和config (provide the config and bsp)

• BSP: bsp/qemu-virt64-riscv, standard RT-Thread (not RT-Smart).
• .config: BSP defaults plus RT_USING_UTESTCASES, RT_USING_CI_ACTION,
  UTEST_THR_STACK_SIZE=8192, and RT_UTEST_{TIMER,OBJECT,THREAD,ATOMIC,SCHEDULER,SEMAPHORE,EVENT,MESSAGEQUEUE,MUTEX,MAILBOX}.
  C++ disabled. Built with GNU RV64 GCC 13.2.0 and Picolibc at
  764ef4e401a8f4c6a86ab723533841f072885a5b, configured for lp64, without
  TLS, multilib, picocrt or semihosting. Set RTT_PICOLIBC_SPECS to the
  absolute installed lib/picolibc.specs path, then run
  scons --pyconfig-silent and scons -j4.
• action: Local ARM64-host build and QEMU validation, not a GitHub Actions run.
  Boot command: qemu-system-riscv64 -nographic -monitor none -M virt -m 256M -smp 1 -bios default -kernel rtthread.bin -nic none.
  All 18 upstream utest cases passed (91 unit invocations), including atomic,
  IPC, scheduler stress, thread and timer cases.
  Separate flag checks covered unset/empty/explicit specs and C++ disabled/enabled;
  default compiler flags and C++ linker flags are preserved. This is not a
  C++ runtime, TLS or RT-Smart validation claim. The build retains the existing
  linker warning about an RWX LOAD segment.

The clang-format and new-BSP checklist items are not applicable: this changes
only two existing Python/SCons build files, not C/C++ sources or a new BSP.

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用clang-format 源码格式化工具确保格式符合RT-Thread代码规范 This PR has been formatted with clang-format and complies with RT-Thread code specification
• 如果是新增bsp, 已经添加ci检查到.github/ALL_BSP_COMPILE.json 详细请参考链接BSP自查