ci: Disable LLVM tests on qemu_cortex_a53

There are currently issues building `qemu_cortex_a53` tests with LLVM.

Revert this commit when the LLVM support for this platform is fixed on
Zephyr side.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

.github/workflows/ci.yml

@@ -1740,7 +1740,8 @@ jobs:
           done
         elif [ "${{ matrix.toolchain }}" == "llvm" ]; then
           # Add AArch64 platforms
-          PLATFORM_ARGS+="-p qemu_cortex_a53 "
+          # FIXME: qemu_cortex_a53 currently cannot build using LLVM.
+          # PLATFORM_ARGS+="-p qemu_cortex_a53 "
 
           # Add ARM platforms
           ## ARMv6-M (Cortex-M0)