Add mcpu flag for future use

This commit adds the --mcpu flag to TC so we can emit LLVM and asm
for different architectures.

Author: nicolasvasilache

tc/core/flags.cc

@@ -53,6 +53,7 @@ DEFINE_string(
     "compiler flags to set when nvcc is used");
 
 // CPU codegen options
+DEFINE_string(mcpu, "", "see llvm's --mcpu");
 DEFINE_bool(llvm_dump_before_opt, false, "Print IR before optimization");
 DEFINE_bool(llvm_dump_after_opt, false, "Print IR after optimization");
 DEFINE_bool(llvm_dump_asm, false, "Print asm");

tc/core/flags.h

@@ -36,7 +36,8 @@ DECLARE_string(cuda_compiler);
 DECLARE_string(llvm_flags);
 DECLARE_string(nvcc_flags);
 
-// llvm codegen
+// CPU codegen options
+DECLARE_string(mcpu);
 DECLARE_bool(llvm_dump_before_opt);
 DECLARE_bool(llvm_dump_after_opt);
 DECLARE_bool(llvm_dump_asm);