perf expr: Allow numbers to be followed by a dot

Metrics like UNC_M_POWER_SELF_REFRESH encode 100 as "100." and
consequently the 100 is treated as a symbol. Alter the regular
expression to allow the dot to be before or after the number.

Note, this passed the pmu-events test as that tests the validity of a
number using strtod rather than lex code. strtod allows the dot after.

Add a test for this behavior.

Fixes: 26226a97 (perf expr: Move expr lexer to flex)
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Clarke <pc@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>