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>