github: clang: Test i386 Interlocked functions

The existing CI tests i686 and x86_64 but not i386. Add a step in the
testcases-clang job that compiles t_intrinc.c with -march=i386 using the
i686 llvm-mingw toolchain.

LLVM's x86 backend does not model the absence of CMPXCHG and XADD on
i386, so without kernel32 fallback in intrin-impl.h it silently lowers
__sync_* builtins to i486+ instructions even with -march=i386. Verify
the object contains no CMPXCHG or XADD instructions to ensure kernel32
fallback is being used. Then link and run the test to confirm the
Interlocked functions return correct values.

Signed-off-by: Jaeden Amero <jaeden@patater.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>

.github/workflows/build.yml

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 847fdce..0f5158a 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -623,6 +623,18 @@
           make -j$(nproc)
           make -j$(nproc) install
           make -j$(nproc) check -k -O --no-print-directory TESTS=
+      - name: Test i386 Interlocked functions
+        if: matrix.arch == 'i686'
+        run: |
+          export PATH=/llvm-mingw/bin:$PATH
+          ${{matrix.arch}}-w64-mingw32-clang -std=c99 -march=i386 -c -o t_intrinc.o mingw-w64-crt/testcases/t_intrinc.c
+          if ${{matrix.arch}}-w64-mingw32-objdump -d t_intrinc.o | grep -qiE 'cmpxchg|xadd'; then
+            echo "Error: i386 object contains i486+ instructions:"
+            ${{matrix.arch}}-w64-mingw32-objdump -d t_intrinc.o | grep -iE 'cmpxchg|xadd'
+            exit 1
+          fi
+          ${{matrix.arch}}-w64-mingw32-clang -std=c99 -march=i386 -o t_intrinc.exe mingw-w64-crt/testcases/t_intrinc.c
+          ./t_intrinc.exe
       - name: Run mingw-w64-crt testcases
         if: matrix.arch != 'armv7'
         run: |