perf env: Adopt perf_header__set_cmdline

perf env: Adopt perf_header__set_cmdline

Move this from two globals to perf_env global, that eventually will
be just perf_header->env or something else, to ease the refactoring
series, leave it as a global and go on reading more of its fields,
not as part of the header writing process but as a perf_env init one
that will be used for perf.data-less situations.

Tested-by: Wang Nan <wangnan0@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-2j78tdf8zn1ci0y6ji15bifj@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>