arch/arm/ameba: fix per-IC asdk toolchain selection and fwlib archive hygiene

[raulcxw]

Summary

Two independent Ameba build-system fixes:

• Per-IC asdk toolchain selection. The asdk (arm-none-eabi) toolchain
  version is declared per SoC-project (component/soc/<soc>/project/CMakeLists.txt: v_ASDK_VER), overriding the top-level cmake/global_define.cmake default. The
  RTL8720F project declares 12.3.1 while the amebadplus (RTL8721Dx) subtree
  default is 10.3.1, but toolchain.mk / ameba_fetch_toolchain.sh only read the
  top-level default, so RTL8720F was built with the wrong toolchain (10.3.1
  instead of its declared 12.3.1). Now the per-SoC CMakeLists is read first (via
  AMEBA_SOC_NAME), falling back to the global default.

• fwlib/wifi archive hygiene. libameba_fwlib.a / libameba_wifi.a were
  archived with a fwlib_obj/*.o wildcard, but distclean does not clean the
  gitignored prebuilt object dir. On an incremental build after the source list
  changed, a stale object (e.g. log.o, whose .bss is ROM-resident) was
  silently re-archived and dragged into the link, tripping the SDK ROM-overlay
  assert (__romlib_bss_end__ is changed). A clean build is unaffected. Now only
  the explicit object list from AMEBA_FWLIB_SRCS / AMEBA_WIFI_SRCS is
  archived, and the archive is removed before rebuilding so it never accumulates.

Impact

• Is new feature added / existing changed? NO (build-system fixes only).
• Impact on user? NO.
• Impact on build? YES — RTL8720F now uses its declared 12.3.1 toolchain, and
  incremental builds no longer trip the ROM-overlay assert from a stale object.
  RTL8721Dx is unaffected (10.3.1, now selected explicitly per-SoC).
• Impact on hardware? NO logic change; affects the arch/arm/ameba build only.
• Impact on documentation? NO.
• Impact on security? NO.
• Impact on compatibility? NO (self-contained to arch/arm/ameba).

Testing

Build Host: Ubuntu (WSL2), x86_64; Realtek asdk arm-none-eabi-gcc auto-fetched by
the board build.
Targets: pke8721daf:nsh (RTL8721Dx), rtl8720f_evb:nsh (RTL8720F).

Before change — incremental RTL8720F build with a stale prebuilt object:

LD: nuttx
arm-none-eabi-ld: __romlib_bss_end__ is changed. Common bss of rom_lib and
rom_tz may be modified illegally.
make[1]: *** [Makefile:230: nuttx] Error 1

After change — both boards build clean; RTL8720F now selects asdk-12.3.1:

# rtl8720f_evb:nsh   -> asdk-12.3.1-4600
PACK: wrote {app.bin, boot.bin, km4ns_image2_all.bin}
# pke8721daf:nsh     -> asdk-10.3.1-4602
PACK: wrote {app.bin, boot.bin, km0_image2_all.bin}

Both images boot to NSH on hardware (boot/iperf logs in the WiFi PR).

PR verification Self-Check

• This PR introduces only one functional change (Ameba build-system).
• I have updated all required description fields above.
• My PR adheres to Contributing Guidelines (commit title/message, coding standard).
• My PR is still work in progress (not ready for review).

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb