Add "-D __BPF_TRACING__" to frontend compilation flags

In 4.17 kernel, x86 build requires compiler asm-goto support. clang
does not support asm-goto and bpf program compilation started to break.
The following kernel commit

  commit b1ae32dbab50ed19cfc16d225b0fb0114fb13025
  Author: Alexei Starovoitov <ast@kernel.org>
  Date:   Sun May 13 12:32:22 2018 -0700

      x86/cpufeature: Guard asm_volatile_goto usage for BPF compilation

      Workaround for the sake of BPF compilation which utilizes kernel
      headers, but clang does not support ASM GOTO and fails the build.

workarounded the issue by permitting native clang compilation.
A warning message, however, is issued:

  ./arch/x86/include/asm/cpufeature.h:150:2: warning: "Compiler lacks ASM_GOTO support.
        Add -D __BPF_TRACING__ to your compiler arguments" [-W#warnings]
  #warning "Compiler lacks ASM_GOTO support. Add -D __BPF_TRACING__ to your compil...
   ^
  1 warning generated.

This patch added "-D __BPF_TRACING__" to clang frontend compilation to
suppress the warning.
Signed-off-by: Yonghong Song <yhs@fb.com>

src/cc/frontends/clang/loader.cc

@@ -147,7 +147,11 @@ int ClangLoader::parse(unique_ptr<llvm::Module> *mod, TableStorage &ts,
   // Enable -O2 for clang. In clang 5.0, -O0 may result in function marking as
   // noinline and optnone (if not always inlining).
   // Note that first argument is ignored in clang compilation invocation.
+  // "-D __BPF_TRACING__" below is added to suppress a warning in 4.17+.
+  // It can be removed once clang supports asm-goto or the kernel removes
+  // the warning.
   vector<const char *> flags_cstr({"-O0", "-O2", "-emit-llvm", "-I", dstack.cwd(),
+                                   "-D", "__BPF_TRACING__",
                                    "-Wno-deprecated-declarations",
                                    "-Wno-gnu-variable-sized-type-not-at-end",
                                    "-Wno-pragma-once-outside-header",