perf llvm: Display eBPF compiling command in debug output

In addition to template, display also the real compile command line with
all the variables substituted.

  llvm compiling command template: $CLANG_EXEC -D__KERNEL__ -D__NR_CPUS__=$NR_CPUS ...
  llvm compiling command : /usr/bin/clang -D__KERNEL__ -D__NR_CPUS__=24 -DLINUX_VERSION_CODE=0x41000 ...

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180312094313.18738-3-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>