bpf: recognize target specific option -mattr=dwarfris in clang

The following is the usage example with clang:
  bash-4.2$ clang -target bpf -O2 -g -c -Xclang -target-feature -Xclang +dwarfris t.c
  bash-4.2$ llvm-objdump -S -d t.o

  t.o:    file format ELF64-BPF

  Disassembly of section .text:
  test:
  ; int test(void) {
       0:       b7 00 00 00 00 00 00 00         r0 = 0
  ; return 0;
         1:       95 00 00 00 00 00 00 00         exit
  bash-4.2$ cat t.c
  int test(void) {
    return 0;
  }
  bash-4.2$

Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 334839