[docs] fix outdated GNU GCC toolchain link in qemu-vexpress-a9 README

[shiptux]

Problem

bsp/qemu-vexpress-a9/README.md tells Linux users to download the GNU GCC
toolchain from a launchpad.net URL. Launchpad shut down its gcc-arm-embedded
downloads years ago — the URL now returns 503 — and the tarball it names is a
2016 Q3 build (gcc 5.4).

Change

• Point the link to Arm GNU Toolchain Downloads, the same page already
  referenced by bsp/nxp/imx/imx91/README.md. The old gnu-rm page covers the
  GNU Arm Embedded Toolchain, which Arm has superseded.
• Mention installing the toolchain from the distribution package manager, with
  Debian/Ubuntu given as one example, since rtconfig.py already defaults
  EXEC_PATH to /usr/bin. libstdc++-arm-none-eabi-newlib is listed because
  LFLAGS links with -lsupc++; binutils-arm-none-eabi is pulled in as a
  dependency of gcc-arm-none-eabi and does not need to be named.
• Note RTT_EXEC_PATH for users who prefer Arm's prebuilt tarball.

This BSP has no English README, so no counterpart file needs the same edit.

[CLAassistant]

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[github-actions]

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