[BPF] do not generate predefined macro bpf

"DefineStd(Builder, "bpf", Opts)" generates the following three
macros:
  bpf
  __bpf
  __bpf__
and the macro "bpf" is due to the fact that the target language
is C which allows GNU extensions.

The name "bpf" could be easily used as variable name or type
field name. For example, in current linux kernel, there are
four places where bpf is used as a field name. If the corresponding
types are included in bpf program, the compilation error will
occur.

This patch removed predefined macro "bpf" as well as "__bpf" which
is rarely used if used at all.

Signed-off-by: Yonghong Song <yhs@fb.com>

Differential Revision: https://reviews.llvm.org/D61173

llvm-svn: 359310