[ARM] Disable llvm's use of threads for armv8-lld-2stage

[DavidSpickett]

Summary

With our current setup using threading leads to the lld tests
requesting so many tests that we hit system resource limits.

https://lab.llvm.org/staging/#/builders/171/builds/65

Long term we need to fix this on our end but for now,
only this 32 bit bot has the problem.

[zatrazz]

LGTM, thanks.

[DavidSpickett]

Landed as https://github.com/llvm/llvm-zorg/commit/88c10a236c3427f239f33d12ee85b571dcba787a.

buildbot/osuosl/master/config/builders.py

'factory' : ClangBuilder.getClangCMakeBuildFactory(
                     runTestSuite=True,
                     testsuite_flags=[
                         '--cppflags', '-mcpu=cortex-a57 -fuse-ld=lld',
                         '--threads=32', '--build-threads=32'],
                     extra_cmake_args=[
                         "-DCMAKE_C_FLAGS='-mcpu=cortex-a57'",
                         "-DCMAKE_CXX_FLAGS='-mcpu=cortex-a57'",
                         "-DCOMPILER_RT_BUILD_SANITIZERS=OFF",
-                        "-DLLVM_ENABLE_LLD=True"])},
+                        "-DLLVM_ENABLE_LLD=True",
+                        # lld tests cause us to hit thread limits
+                        "-DDLLVM_ENABLE_THREADS=OFF"])},
 
     # AArch64 check-all + flang + compiler-rt + test-suite 2-stage
     {'name' : "clang-aarch64-full-2stage",
     'tags'  : ["clang"],
     'workernames' : ["linaro-clang-aarch64-full-2stage"],
     'builddir': "clang-aarch64-full-2stage",
     'factory' : ClangBuilder.getClangCMakeBuildFactory(
                     clean=False,