Fix stats printing for floats and negative deltas

Ratio in YJIT might print something like this:
 #8: 1918ms  0.305937303071417
 #9: 1950ms -0.396978503607869

harness/harness-common.rb

@@ -114,7 +114,7 @@ def return_results(warmup_iterations, bench_iterations)
   if yjit_stats
     yjit_bench_results["yjit_stats"] = yjit_stats
 
-    formatted_stats = proc { |key| "%10s" % yjit_stats[key].to_s.reverse.scan(/\d{1,3}/).join(",").reverse }
+    formatted_stats = proc { |key| "%10s" % yjit_stats[key].to_s.split(".").tap { |a| a[0] = a[0].reverse.scan(/\d{1,3}/).join(",").reverse }.join(".") }
     yjit_stats_keys = [
       *ENV.fetch("YJIT_BENCH_STATS", "").split(",").map(&:to_sym),
       :inline_code_size,

harness/harness.rb

@@ -59,7 +59,16 @@ def run_benchmark(_num_itrs_hint, &block)
 
     yjit_stats&.each do |key, old_value|
       new_value = RubyVM::YJIT.runtime_stats(key)
-      diff = (new_value - old_value).to_s.reverse.scan(/\d{1,3}/).join(",").reverse
+
+      diff = (new_value - old_value).to_s.tap do |s|
+        # Insert comma separators but only in the whole number portion.
+        i = s.index('.') || s.size
+        s.insert(i -= 3, ',') while i > 3
+        # Add a space when positive so that if there is ever a negative
+        # the first digit will line up.
+        s.prepend(" ") unless s.start_with?("-")
+      end
+
       itr_str << " %#{key.size}s" % diff
       yjit_stats[key] = new_value
     end