workaround kernel 5.0 asm_volatile_goto issue (#2133)

Fix issue #2119.

Kernel 5.0 added more usages of asm goto and llvm
does not support asm goto yet. This resulted in
compilation error for virtually any bcc scripts.

The workaround here is to redefine asm_volatile_goto to
  asm volatile("invalid use of asm_volatile_goto")
which can pass clang.
If bpf program does not use asm_volatile_goto,
nothing bad will happen. The functions using
asm_volatile_goto will be thrown away.
If bpf program accidentally uses asm_volatile_goto,
a compilation error like below will be printed out:
  <inline asm>:1:2: error: invalid register/token name
          invalid use of asm_volatile_goto
          ^
  LLVM ERROR: Error parsing inline asm
Signed-off-by: Yonghong Song <yhs@fb.com>

src/cc/export/helpers.h

@@ -17,6 +17,21 @@ R"********(
 #ifndef __BPF_HELPERS_H
 #define __BPF_HELPERS_H
 
+/* Before bpf_helpers.h is included, uapi bpf.h has been
+ * included, which references linux/types.h. This will bring
+ * in asm_volatile_goto definition if permitted based on
+ * compiler setup and kernel configs.
+ *
+ * clang does not support "asm volatile goto" yet.
+ * So redefine asm_volatile_goto to some invalid asm code.
+ * If asm_volatile_goto is actually used by the bpf program,
+ * a compilation error will appear.
+ */
+#ifdef asm_volatile_goto
+#undef asm_volatile_goto
+#define asm_volatile_goto(x...) asm volatile("invalid use of asm_volatile_goto")
+#endif
+
 #include <uapi/linux/bpf.h>
 #include <uapi/linux/if_packet.h>
 #include <linux/version.h>