Replace KB unit with KiB (#4293)

kB (*kilo* byte) unit means 1000 bytes, whereas KiB ("kibibyte")
means 1024 bytes. KB was misused: replace kB or KB with KiB when
appropriate.

Same change for MB and GB which become MiB and GiB.

Change the output of Tools/iobench/iobench.py.

Round also the size of the documentation from 5.5 MB to 5 MiB.

cygwinccompiler.py

@@ -234,8 +234,8 @@ class CygwinCCompiler(UnixCCompiler):
         # who wants symbols and a many times larger output file
         # should explicitly switch the debug mode on
         # otherwise we let dllwrap/ld strip the output file
-        # (On my machine: 10KB < stripped_file < ??100KB
-        #   unstripped_file = stripped_file + XXX KB
+        # (On my machine: 10KiB < stripped_file < ??100KiB
+        #   unstripped_file = stripped_file + XXX KiB
         #  ( XXX=254 for a typical python extension))
         if not debug:
             extra_preargs.append("-s")