perf llvm: Allow passing options to llc in addition to clang

The newly added 'llvm.opts' variable allows passing options directly to
llc, like needed to get sane DWARF in BPF ELF debug sections:

With:

  [root@seventh perf]# cat ~/.perfconfig
  [llvm]
	  dump-obj = true
	clang-opt = -g
  [root@seventh perf]#

We get:

  [root@seventh perf]# perf trace -e tools/perf/examples/bpf/hello.c cat /etc/passwd > /dev/null
  LLVM: dumping tools/perf/examples/bpf/hello.o
       0.000 __bpf_stdout__:Hello, world
       0.015 __bpf_stdout__:Hello, world
       0.187 __bpf_stdout__:Hello, world
  [root@seventh perf]# pahole tools/perf/examples/bpf/hello.o
  struct clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c) {
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566e.org clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*     0     4 */
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566e.org clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*     4     4 */
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566e.org clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*     8     4 */
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566e.org clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*    12     4 */
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566e.org clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*    16     4 */
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566e.org clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*    20     4 */
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566e.org clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*    24     4 */

	  /* size: 28, cachelines: 1, members: 7 */
	  /* last cacheline: 28 bytes */
  };
  [root@seventh perf]#

Adding these options to be passed to llvm's llc:

  [root@seventh perf]# cat ~/.perfconfig
  [llvm]
	  dump-obj = true
	  clang-opt = -g
	  opts = -mattr=dwarfris
  [root@seventh perf]#

We get sane output:

  [root@seventh perf]# perf trace -e tools/perf/examples/bpf/hello.c cat /etc/passwd > /dev/null
  LLVM: dumping tools/perf/examples/bpf/hello.o
       0.000 __bpf_stdout__:Hello, world
       0.015 __bpf_stdout__:Hello, world
       0.185 __bpf_stdout__:Hello, world
  [root@seventh perf]# pahole tools/perf/examples/bpf/hello.o
  struct bpf_map {
	  unsigned int               type;                 /*     0     4 */
	  unsigned int               key_size;             /*     4     4 */
	  unsigned int               value_size;           /*     8     4 */
	  unsigned int               max_entries;          /*    12     4 */
	  unsigned int               map_flags;            /*    16     4 */
	  unsigned int               inner_map_idx;        /*    20     4 */
	  unsigned int               numa_node;            /*    24     4 */

	  /* size: 28, cachelines: 1, members: 7 */
	  /* last cacheline: 28 bytes */
  };
  [root@seventh perf]#

Cc: Alexei Starovoitov <ast@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>,
Cc: Yonghong Song <yhs@fb.com>
Link: https://lkml.kernel.org/n/tip-0lrwmrip4dru1651rm8xa7tq@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>