[clang] [Driver] Enable outline atomics for OpenBSD/aarch64 (PR #155713)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-quick` running on `linaro-clang-aarch64-quick` while building `clang` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/65/builds/21880

<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 'Clangd Unit Tests :: ./ClangdTests/243/332' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests-Clangd Unit Tests-2231171-243-332.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=332 GTEST_SHARD_INDEX=243 /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests
--

Note: This is test shard 244 of 332.
[==========] Running 4 tests from 4 test suites.
[----------] Global test environment set-up.
[----------] 1 test from CompletionStringTest
[ RUN      ] CompletionStringTest.Documentation
[       OK ] CompletionStringTest.Documentation (0 ms)
[----------] 1 test from CompletionStringTest (0 ms total)

[----------] 1 test from FuzzyMatch
[ RUN      ] FuzzyMatch.Matches
[       OK ] FuzzyMatch.Matches (18 ms)
[----------] 1 test from FuzzyMatch (19 ms total)

[----------] 1 test from CrossFileRenameTests
[ RUN      ] CrossFileRenameTests.WithUpToDateIndex
ASTWorker building file /clangd-test/foo.h version null with command 
[/clangd-test]
clang -xobjective-c++ /clangd-test/foo.h
Driver produced command: cc1 -cc1 -triple aarch64-unknown-linux-gnu -fsyntax-only -disable-free -clear-ast-before-backend -main-file-name foo.h -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=non-leaf -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -enable-tlsdesc -target-cpu generic -target-feature +v8a -target-feature +fp-armv8 -target-feature +neon -target-abi aapcs -debugger-tuning=gdb -fdebug-compilation-dir=/clangd-test -fcoverage-compilation-dir=/clangd-test -resource-dir lib/clang/22 -internal-isystem lib/clang/22/include -internal-isystem /usr/local/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdeprecated-macro -ferror-limit 19 -fno-signed-char -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fobjc-runtime=gcc -fobjc-encode-cxx-class-template-spec -fobjc-exceptions -fcxx-exceptions -fexceptions -no-round-trip-args -target-feature -fmv -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -x objective-c++ /clangd-test/foo.h
Building first preamble for /clangd-test/foo.h version null
Built preamble of size 820840 for file /clangd-test/foo.h version null in 9.08 seconds
indexed preamble AST for /clangd-test/foo.h version null:
  symbol slab: 0 symbols, 120 bytes
  ref slab: 0 symbols, 0 refs, 128 bytes
  relations slab: 0 relations, 24 bytes
not idle after addDocument
UNREACHABLE executed at ../llvm/clang-tools-extra/clangd/unittests/SyncAPI.cpp:22!
indexed file AST for /clangd-test/foo.h version null:
  symbol slab: 3 symbols, 4912 bytes
  ref slab: 3 symbols, 5 refs, 4320 bytes
  relations slab: 0 relations, 24 bytes
Build dynamic index for main-file symbols with estimated memory usage of 12648 bytes
 #0 0x0000c19fd0224bdc llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xc64bdc)
 #1 0x0000c19fd02226a4 llvm::sys::RunSignalHandlers() (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xc626a4)
 #2 0x0000c19fd0225a38 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
 #3 0x0000ff5c76dfe8f8 (linux-vdso.so.1+0x8f8)
 #4 0x0000ff5c7695f1f0 __pthread_kill_implementation ./nptl/./nptl/pthread_kill.c:44:76
 #5 0x0000ff5c7691a67c gsignal ./signal/../sysdeps/posix/raise.c:27:6
 #6 0x0000ff5c76907130 abort ./stdlib/./stdlib/abort.c:81:7
 #7 0x0000c19fd01d1484 llvm::RTTIRoot::anchor() (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xc11484)
 #8 0x0000c19fd007c64c clang::clangd::runCodeComplete(clang::clangd::ClangdServer&, llvm::StringRef, clang::clangd::Position, clang::clangd::CodeCompleteOptions) (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xabc64c)
 #9 0x0000c19fcffc8778 clang::clangd::(anonymous namespace)::CrossFileRenameTests_WithUpToDateIndex_Test::TestBody() RenameTests.cpp:0:0
#10 0x0000c19fd027d078 testing::Test::Run() (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xcbd078)
#11 0x0000c19fd027e39c testing::TestInfo::Run() (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xcbe39c)
...

```

</details>

https://github.com/llvm/llvm-project/pull/155713