Add toolchain reference documentation

Author: widgetii

README.md

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+# HiSilicon Cross-Compilation Toolchains
+
+Cross-compiler toolchains extracted from HiSilicon SDKs, hosted as release assets for use in CI pipelines.
+
+## Toolchain Reference
+
+| Toolchain | GCC | Libc | Float | Default arch | Multilib targets | `CROSS_COMPILE` | Source SDKs |
+|-----------|-----|------|-------|-------------|-----------------|-----------------|-------------|
+| arm-hisiv300-linux | 4.8.3 (Linaro) | uClibc 0.9.33.2 | softfp | armv5te | ARM926, A9, A7 | `arm-hisiv300-linux-uclibcgnueabi-` | Hi3518E_SDK_V1.0.4.0, Hi3516A_SDK_V1.0.5.0 |
+| arm-hisiv500-linux | 4.9.4 (Linaro) | uClibc 0.9.33.2 | softfp | armv5te | ARM926, A9, A7 | `arm-hisiv500-linux-uclibcgnueabi-` | Hi3516CV300_SDK, Hi3516C V300R001C01SPC040 |
+| arm-hisiv510-linux | 6.2.1 (Linaro) | uClibc 0.9.33.2 | softfp | armv5te | ARM926, A9, A7, A17, A53, A73, M7 | `arm-hisiv510-linux-uclibcgnueabi-` | Hi3518E V200R001C01SPC050, Hi3516A V100R001C01SPC080 |
+| arm-hisiv600-linux | 4.9.4 (Linaro) | glibc 2.20 | softfp | armv5te | ARM926, A9, A7 | `arm-hisiv600-linux-gnueabi-` | Hi3516C V300R001C02SPC040 |
+| arm-himix100-linux | 6.3.0 | uClibc 0.9.33.2 | softfp | armv5te | ARM926, A9, A7, A17, A53, A73 | `arm-himix100-linux-` | Hi3516EV200 series |
+| arm-himix200-linux | 6.3.0 | glibc 2.24 | softfp | armv5te | ARM926, A9, A7, A17, A53, A73 | `arm-himix200-linux-` | Hi3516CV500, Hi3519V101 |
+
+All toolchains default to soft-float ABI (gnueabi). Hard-float variants are available via multilib flags, e.g.:
+
+```sh
+-mcpu=cortex-a7 -mfloat-abi=hard -mfpu=neon-vfpv4
+```
+
+## Archive Format
+
+Most archives are `.tgz` files with a nested structure:
+
+```
+<name>.tgz
+  +-- <name>/
+      +-- <name>.tar.bz2    # actual toolchain
+      +-- <name>.install     # vendor install script (not needed for CI)
+      +-- runtime_*.tgz     # runtime libraries
+```
+
+**Exceptions:**
+
+- `arm-hisiv300-linux.tar.bz2` is the inner archive directly (no `.tgz` wrapper).
+- `arm-himix200-linux.tgz` is split into two parts due to the 2 GB GitHub release asset limit.
+
+## Usage in CI
+
+### Extracting
+
+```sh
+# Standard .tgz archives (two-step extraction)
+tar xzf arm-hisiv510-linux.tgz
+tar xjf arm-hisiv510-linux/arm-hisiv510-linux.tar.bz2
+
+# arm-hisiv300 (single-step, bare tar.bz2)
+tar xjf arm-hisiv300-linux.tar.bz2
+
+# arm-himix200 (reassemble split parts first)
+cat arm-himix200-linux.tgz.part* > arm-himix200-linux.tgz
+tar xzf arm-himix200-linux.tgz
+tar xjf arm-himix200-linux/arm-himix200-linux.tar.bz2
+```
+
+### Building
+
+The vendor install script creates `target/bin/` with absolute symlinks to `/opt/hisi-linux/...` which only work on the original install path. For CI, use the full `CROSS_COMPILE` prefix (from the table above) with the `bin/` directory on `PATH`:
+
+```sh
+export PATH="$PWD/arm-hisiv510-linux/bin:$PATH"
+make CROSS_COMPILE=arm-hisiv510-linux-uclibcgnueabi-
+```
+
+### Downloading in GitHub Actions
+
+```yaml
+- name: Download toolchain
+  env:
+    GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+  run: gh release download v1 -R OpenIPC/toolchains -p 'arm-hisiv510-linux.tgz'
+```