[AMDGPU] Split GCNSubtarget into its own file. NFC. (#105525)

Author: jayfoad

llvm/lib/Target/AMDGPU/AMDGPUSubtarget.cpp

@@ -119,6 +119,7 @@ add_llvm_target(AMDGPUCodeGen
   GCNRegPressure.cpp
   GCNRewritePartialRegUses.cpp
   GCNSchedStrategy.cpp
+  GCNSubtarget.cpp
   GCNVOPDUtils.cpp
   R600AsmPrinter.cpp
   R600ClauseMergePass.cpp