perf tests: Make x86 new instructions test optional at build time

The "x86 instruction decoder - new instructions" test takes up space but
is only really useful to developers. Make it optional at build time.

Add variable EXTRA_TESTS which must be defined in order to build perf
with the test.

Example:

  Before:

    $ make -C tools/perf clean >/dev/null
    $ make -C tools/perf >/dev/null
    Makefile.config:650: No libunwind found. Please install libunwind-dev[el] >= 1.1 and/or set LIBUNWIND_DIR
    Makefile.config:1149: libpfm4 not found, disables libpfm4 support. Please install libpfm4-dev
      PERF_VERSION = 6.4.rc3.gd15b8c76c964
    $ readelf -SW tools/perf/perf | grep '\.rela.dyn\|.rodata\|\.data.rel.ro'
      [10] .rela.dyn         RELA            000000000002fcb0 02fcb0 0748b0 18   A  6   0  8
      [18] .rodata           PROGBITS        00000000002eb000 2eb000 6bac00 00   A  0   0 32
      [25] .data.rel.ro      PROGBITS        00000000009ea180 9e9180 04b540 00  WA  0   0 32

  After:

    $ make -C tools/perf clean >/dev/null
    $ make -C tools/perf >/dev/null
    Makefile.config:650: No libunwind found. Please install libunwind-dev[el] >= 1.1 and/or set LIBUNWIND_DIR
    Makefile.config:1154: libpfm4 not found, disables libpfm4 support. Please install libpfm4-dev
      PERF_VERSION = 6.4.rc3.g4ea9c1569ea4
    $ readelf -SW tools/perf/perf | grep '\.rela.dyn\|.rodata\|\.data.rel.ro'
      [10] .rela.dyn         RELA            000000000002f3c8 02f3c8 036d68 18   A  6   0  8
      [18] .rodata           PROGBITS        00000000002ac000 2ac000 68da80 00   A  0   0 32
      [25] .data.rel.ro      PROGBITS        000000000097d440 97c440 022280 00  WA  0   0 32

Committer notes:

Build with 'make EXTRA_TESTS=1 -C tools/perf O=/tmp/build/perf" and
reproduced the ELF section size differences.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/683fea7c-f5e9-fa20-f96b-f6233ed5d2a7@intel.com


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