[AMDGPU] Always pass -mcpu to the lld linker

Currently, AMDGPU more or less only supports linking with LTO. If the
user does not either pass `-flto` or `-Wl,-plugin-opt=mcpu=` manually
linking will fail because the architecture's aren't compatible. THis
patch simply passes `-mcpu` by default if it was specified. Should be a
no-op if it's not actually used.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D153909

clang/lib/Driver/ToolChains/AMDGPU.cpp

@@ -545,6 +545,9 @@ void amdgpu::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   if (C.getDriver().isUsingLTO())
     addLTOOptions(getToolChain(), Args, CmdArgs, Output, Inputs[0],
                   C.getDriver().getLTOMode() == LTOK_Thin);
+  else if (Args.hasArg(options::OPT_mcpu_EQ))
+    CmdArgs.push_back(Args.MakeArgString(
+        "-plugin-opt=mcpu=" + Args.getLastArgValue(options::OPT_mcpu_EQ)));
   CmdArgs.push_back("--no-undefined");
   CmdArgs.push_back("-shared");
   CmdArgs.push_back("-o");

clang/test/Driver/amdgpu-toolchain.c

@@ -11,6 +11,8 @@
 // DWARF_VER: "-dwarf-version=5"
 
 // RUN: %clang -### --target=amdgcn-amd-amdhsa -mcpu=gfx906 -nogpulib \
-// RUN:   -L. -flto -fconvergent-functions %s 2>&1 | FileCheck -check-prefix=LTO %s
+// RUN:   -L. -flto -fconvergent-functions %s 2>&1 | FileCheck -check-prefixes=LTO,MCPU %s
+// RUN: %clang -### --target=amdgcn-amd-amdhsa -mcpu=gfx906 -nogpulib \
+// RUN:   -L. -fconvergent-functions %s 2>&1 | FileCheck -check-prefix=MCPU %s
 // LTO: clang{{.*}} "-flto=full"{{.*}}"-fconvergent-functions"
-// LTO: ld.lld{{.*}}"-L."{{.*}}"-plugin-opt=mcpu=gfx906"
+// MCPU: ld.lld{{.*}}"-L."{{.*}}"-plugin-opt=mcpu=gfx906"