`get_host_triple()` hard-fails on HarmonyOS because `config.guess` cannot identify the system

## Summary

On HarmonyOS, `uname -s` reports `HarmonyOS` (not `Linux`), and the `config.guess` script cannot determine the system, so CMake configuration fails. The workaround is to hard-code the host triple.

This is fundamentally a `config.guess` gap, not an LLVM bug — the LLVM angle is just that `GetHostTriple.cmake` has no OHOS special-case to avoid `config.guess`.

## Detailed description

Configuring LLVM (`cmake -S llvm` and later `cmake -S runtimes`) on HarmonyOS 7 invokes `llvm/cmake/modules/GetHostTriple.cmake`, which then invokes `config.guess` to determine the system. However, `config.guess`'s attempt to create a dummy file for probing will be rejected by HarmonyOS's sandbox mechanisms. Even if we set `TMPDIR` to a safe place to avoid this issue, it still fails to recognize the `uname`'s outputs:

```
localhost ~ % TMPDIR=$(brew --cache)/tmp sh config.guess
config.guess: unable to guess system type

This script (version 2026-05-17), has failed to recognize the
operating system you are using. If your script is old, overwrite *all*
copies of config.guess and config.sub with the latest versions from:

https://git.savannah.gnu.org/cgit/config.git/plain/config.guess
and
https://git.savannah.gnu.org/cgit/config.git/plain/config.sub

If config.guess has already been updated, send the following data and any
information you think might be pertinent to config-patches@gnu.org to
provide the necessary information to handle your system.

config.guess timestamp = 2026-05-17

uname -m = aarch64
uname -r = HongMeng Kernel 1.13.0
uname -s = HarmonyOS
uname -v = #1 SMP Sat Aug 15 11:19:26 UTC 2026

/usr/bin/uname -p = unknown
/bin/uname -X =

hostinfo =
/bin/universe =
/usr/bin/arch -k =
/bin/arch =
/usr/bin/oslevel =
/usr/convex/getsysinfo =

UNAME_MACHINE = "aarch64"
UNAME_RELEASE = "HongMeng Kernel 1.13.0"
UNAME_SYSTEM = "HarmonyOS"
UNAME_VERSION = "#1 SMP Sat Aug 15 11:19:26 UTC 2026"
```

This is primarily a gap belonging to `config.guess`, not actually LLVM's problem. As a workaround, we simply overwrite the `get_host_triple` function to hardcode the value (`aarch64-unknown-linux-ohos`, which is obtained from `clang -dumpmachine`). But `GetHostTriple.cmake` could still special-case `OHOS` and `HarmonyOS` the same way it already special-cases Windows, MinGW, z/OS and AIX, i.e., returning `aarch64-*-linux-ohos` based on CMake variables instead of falling through to `config.guess`.

---

We used branch `release/22.x` commit `ca7933e47d3a3451d81e72ac174dcb5aa28b59d1`.

Environment: HarmonyOS 7.0.0.102, `uname -a`: `HarmonyOS localhost HongMeng Kernel 1.13.0 #1 SMP Sat Aug 15 11:19:26 UTC 2026 aarch64 GNU/Linux`

Start with llvm/cmake/modules/GetHostTriple.cmake and compare its existing platform-specific handling with the HarmonyOS and OHOS details in the report. Test configuration for both llvm and runtimes on HarmonyOS, using the reported aarch64 environment, and verify that host-triple detection no longer falls through to the failing config.guess path.

Written by the indexing model from the issue text.