[gn build] Port a6bae5c

Author: llvmgnsyncbot

llvm/utils/gn/secondary/llvm/lib/Target/AMDGPU/BUILD.gn

@@ -206,6 +206,7 @@ static_library("LLVMAMDGPUCodeGen") {
     "GCNRegPressure.cpp",
     "GCNRewritePartialRegUses.cpp",
     "GCNSchedStrategy.cpp",
+    "GCNSubtarget.cpp",
     "GCNVOPDUtils.cpp",
     "R600AsmPrinter.cpp",
     "R600ClauseMergePass.cpp",