[llvm] [Offload] Fix PowerPC builds that pass -mcpu (PR #138327)

[Shilei Tian via llvm-commits]

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@@ -132,9 +132,12 @@ function(compileDeviceRTLLibrary target_name target_triple)
     BUILD_RPATH ""
     INSTALL_RPATH ""
     RUNTIME_OUTPUT_NAME libomptarget-${target_name}.bc)
-  target_compile_options(libomptarget-${target_name} PRIVATE "--target=${target_triple}" "-fuse-ld=lld" "-march=")
+  target_compile_options(libomptarget-${target_name} PRIVATE
+    "--target=${target_triple}" "-fuse-ld=lld" "-march=" "-mcpu="
+    "-Wno-unused-command-line-argument")
   target_link_options(libomptarget-${target_name} PRIVATE "--target=${target_triple}"
-                      "-r" "-nostdlib" "-flto" "-Wl,--lto-emit-llvm" "-fuse-ld=lld" "-march=")
+                      "-mcpu=power8" "-r" "-nostdlib" "-flto" "-Wl,--lto-emit-llvm"
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shiltian wrote:

Why can we hard code `power8` here?

https://github.com/llvm/llvm-project/pull/138327