toolchain: drop legacy gcc 8.4 pin on 18 ARM platforms

[widgetii]

Summary

Drop the explicit `# Legacy` block from 18 defconfigs:

```

• Legacy

• BR2_TOOLCHAIN_EXTERNAL_GCC_8=y
• BR2_GCC_VERSION_LEGACY=y
  ```

Without an explicit pin they fall through to `BR2_GCC_VERSION_13_X` (Buildroot 2024.02.10 default per `general/package/gcc/Config.in.host`), so cross-toolchains now build with gcc 13.3.0 instead of 8.4.0.

Platforms affected

Vendor	Platforms
Goke (11)	gk7102, gk7102s, gk7202v300_{lite,ultimate}, gk7205v200_{lite,original,ultimate}, gk7205v210_lite, gk7205v300_{lite,ultimate}, gk7605v100_lite
Grainmedia (2)	gm8135_lite, gm8136_lite
Novatek (2)	nt98562_lite, nt98566_lite
Xiongmai (3)	xm510_lite, xm530_lite, xm550_lite

All 18 are ARM (oldest core is arm1176jzf_s on the gk7102 family) — well within gcc 13's supported set.

Why now

The legacy pin originated in #1726 ("[no ci] Set legacy GCC version for Goke", 2023) and propagated. Upstream gcc, musl, and libsanitizer have since caught up:

• all-patches/gcc: scope libsanitizer-musl-compat patch to gcc 11+ #2161 just scoped the libsanitizer-musl-compat patch to gcc 11/12/13 subdirs (away from gcc 8.4), but the underlying root cause is that the gcc 8.4 path has been growing crustier — the `.cc` → `.cpp` source rename was just one symptom.
• Folding all platforms onto a single host gcc cuts toolchain compile time, removes a class of "works in gcc 11+, broken on gcc 8" patch-scoping bugs going forward, and shrinks the matrix of variants the project has to validate against musl.

Test plan

• Wait for CI's `Firmware (…)` matrix to land — each of the 18 affected platforms will rebuild its toolchain on first hit (the wget-spider skip-check sees the existing tarballs but those were built with gcc 8.4; this PR doesn't itself force a re-roll, so first verification is the firmware build step exercising the existing-with-old-gcc external toolchain — that should still link cleanly because the toolchain ABI hasn't changed).
• After merge, run `gh release delete-asset toolchain --yes` for each of the 18 platforms and dispatch `toolchain.yml` to materially re-roll them with gcc 13. Verify they actually build under the new compiler.
• Smoke a flash on one Goke camera (gk7205v200 lab unit) and one Xiongmai (xm530) to confirm runtime parity.

Out of scope

This PR does not bump kernel-headers selections (`BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_*` for the Xiongmai 3.0/3.4 etc. stays as is) — those are independent of the host compiler version.