[Clang] Use x86 triple for offload-Xarch.c test

This fixes an issue where your host triple is not compatible with
the 64 bit ptx being the offload architecture. At least, that's my
guess.

This failed on our Arm 32 bit bot:
https://lab.llvm.org/buildbot/#/builders/154/builds/11413/steps/5/logs/FAIL__Clang__offload-Xarch_c

Crucially it outputted:
clang: warning: argument unused during compilation: '-Xarch_nvptx64 -O3' [-Wunused-command-line-argument]

Making the triple always something 64 bit means this will work everywhere.

Fixes 455cedc / llvm#125421.

Author: DavidSpickett

clang/test/Driver/offload-Xarch.c

@@ -1,4 +1,4 @@
-// RUN: %clang -x cuda %s -Xarch_nvptx64 -O3 -S -nogpulib -nogpuinc -### 2>&1 | FileCheck -check-prefix=O3ONCE %s
+// RUN: %clang --target=x86_64-unknown-linux-gnu -x cuda %s -Xarch_nvptx64 -O3 -S -nogpulib -nogpuinc -### 2>&1 | FileCheck -check-prefix=O3ONCE %s
 // RUN: %clang -x cuda %s -Xarch_device -O3 -S -nogpulib -nogpuinc -### 2>&1 | FileCheck -check-prefix=O3ONCE %s
 // RUN: %clang -x hip %s -Xarch_amdgcn -O3 -S -nogpulib -nogpuinc -### 2>&1 | FileCheck -check-prefix=O3ONCE %s
 // RUN: %clang -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa -nogpulib -nogpuinc \