CI: Run 128-bit SVE tests natively on dedicated ARM runner

128-bit SVE matches native ARMv9 hardware (Neoverse V2 / Google Axion)
and can execute directly in silicon on our dedicated ARM runner.

- In ci/build.linux.gitlab-ci.yml, remove CMAKE_CROSSCOMPILING_EMULATOR
  from build:linux:cross:sve:gcc-13:vl128 so CTest does not prefix test
  targets with qemu-aarch64.
- In ci/test.linux.gitlab-ci.yml, update .test:linux:sve:vl128 to extend
  .test:linux directly (dropping QEMU TCG timeout overrides and test
  exclusions), install native g++-13, and target tags [eigen-runner,
  linux, aarch64, sve].
- Retain synthetic 256-bit and 512-bit vector length tests under QEMU
  on SaaS runners.