Remove external Linaro download & add Automake/Autotools to Docker image

[quic-mtharu]

Summary

This PR simplifies and hardens our CI Docker image by:

1. Removing the Linaro GCC 7.5 download/extract step (was using latest-7 and i686 host path,
   which is unstable and not needed).
2. Installing Automake/Autotools (autoconf, automake, libtool, autoconf-archive, pkg-config)
   so autoreconf -fi and aclocal work for projects that use Autotools.

Why this change?

• The Linaro URL is brittle (redirects/404s) and targeted the wrong host (i686). We already install
  Ubuntu’s cross toolchain (gcc-aarch64-linux-gnu, g++-aarch64-linux-gnu, libc6-dev-arm64-cross)
  which is sufficient for our aarch64 builds and far more reproducible.
• Autotools are currently missing in the container, causing fastrpc-image-run ./gitcompile /
  autoreconf to fail with aclocal: command not found and missing config.guess/config.sub.

Impact & Benefits

• Reliability: Eliminates an external network dependency and the recurring exit code 4 failures
  during wget/tar of the Linaro tarball.
• Reproducibility: Uses distro toolchain packages instead of a mutable latest-7 URL.
• Developer productivity: Autotools‑based projects now bootstrap cleanly (autoreconf -fi),
  unblocking PR CI without manual intervention.

Validation

• Built the image locally/CI without the Linaro step; cross‑compiler from Ubuntu works as expected.
• Verified Autotools flow: autoreconf -fi && ./configure --host=aarch64-linux-gnu && make
  runs successfully in the container.