[clang] [Clang] Add __builtin_bswapg (PR #162433)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-armv8-quick` running on `linaro-clang-armv8-quick` while building `clang` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/154/builds/24121

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'Clang :: CodeGen/builtins.c' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
/home/tcwg-buildbot/worker/clang-armv8-quick/stage1/bin/clang -cc1 -internal-isystem /home/tcwg-buildbot/worker/clang-armv8-quick/stage1/lib/clang/22/include -nostdsysteminc -emit-llvm -o /home/tcwg-buildbot/worker/clang-armv8-quick/stage1/tools/clang/test/CodeGen/Output/builtins.c.tmp /home/tcwg-buildbot/worker/clang-armv8-quick/llvm/clang/test/CodeGen/builtins.c
# executed command: /home/tcwg-buildbot/worker/clang-armv8-quick/stage1/bin/clang -cc1 -internal-isystem /home/tcwg-buildbot/worker/clang-armv8-quick/stage1/lib/clang/22/include -nostdsysteminc -emit-llvm -o /home/tcwg-buildbot/worker/clang-armv8-quick/stage1/tools/clang/test/CodeGen/Output/builtins.c.tmp /home/tcwg-buildbot/worker/clang-armv8-quick/llvm/clang/test/CodeGen/builtins.c
# .---command stderr------------
# | /home/tcwg-buildbot/worker/clang-armv8-quick/llvm/clang/test/CodeGen/builtins.c:1292:33: error: __int128 is not supported on this target
# |  1292 |                        unsigned __int128 ui128, _BitInt(8) bi8,
# |       |                                 ^
# | 1 error generated.
# `-----------------------------
# error: command failed with exit status: 1

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/162433