Introducing FEATURES_DUMP make variable to provide features
detection dump file and bypass the feature detection.

The intention is to use this during build tests to skip
repeated features detection, like:

Get feature dump static build into /tmp/fd file:
  $ make feature-dump FEATURE_DUMP_COPY=/tmp/fd LDFLAGS=-static
    BUILD:   Doing 'make -j4' parallel build

  Auto-detecting system features:
  ...                         dwarf: [ OFF ]

  SNIP

  FEATURE-DUMP file copied into /tmp/fd

Use /tmp/fd to build perf:
  $ make FEATURES_DUMP=/tmp/fd LDFLAGS=-static

  $ file perf
  perf: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), statically linked, for ...

Suggested-by: Wang Nan <wangnan0@huawei.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1452830421-77757-7-git-send-email-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>