perf expr: Move expr lexer to flex

Adding expr flex code instead of the manual parser code. So it's easily
extensible in upcoming changes.

The new flex code is in flex.l object and gets compiled like all the
other flexers we use.  It's defined as flex reentrant parser.

It's used by both expr__parse and expr__find_other interfaces by
separating the starting point.

There's no intended change of functionality ;-) the test expr is
passing.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Link: http://lore.kernel.org/lkml/20200228093616.67125-3-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>