tools/impact: Set locale to C at the beginning

Without this, shell's printf is unreliable. For example, on OSX with
cs_CZ.UTF-8 locale it complained about number formatting:

  tools/impact: line 51: printf: .0300: invalid number

tools/impact

@@ -4,6 +4,7 @@
 # speed and size. Makes sense to use only on PEG.js Git repository checkout.
 
 set -e
+export LC_ALL=C
 
 # Measurement