bpf: add -mcpu=# support for bpf

-mcpu=# will support:
  . generic: the default insn set
  . v1: insn set version 1, the same as generic
  . v2: insn set version 2, version 1 + additional jmp insns
  . probe: the compiler will probe the underlying kernel to
           decide proper version of insn set.

Examples:
$ clang -target bpf -mcpu=v1 -c t.c
$ clang -target bpf -mcpu=v2 -c t.c
$ clang -target bpf -mcpu=generic -c t.c
$ clang -target bpf -mcpu=probe -c t.c
$ clang -target bpf -mcpu=v3 -c t.c
error: unknown target CPU 'v3'

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
llvm-svn: 311523