perf expr: Allow exponents on floating point values

perf expr: Allow exponents on floating point values

Pass the optional exponent component through to strtod that already
supports it. We already have exponents in ScaleUnit and so this adds
uniformity.

Reported-by: Zhengjun Xing <zhengjun.xing@linux.intel.com>
Reviewed-By: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.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>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/r/20220527020653.4160884-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>