Stop using .com extension in monorepo

The WIN32 CreateProcess() function does not require an .exe or .com
suffix in order to spawn an executable. Now that we have Cosmo bash
we're no longer so dependent on the cmd.exe prompt.

tool/cosmocc/bin/cosmocross

@@ -163,13 +163,6 @@ for x; do
     # this toolchain is intended for building other people's code
     # elevating warnings into errors, should only be done by devs
     continue
-  elif [ x"$x" = x"-march=native" ]; then
-    if [ $CROSS -eq 0 ]; then
-      set -- "$@" $("$BIN/march-native")
-    else
-      fatal_error "-march=native can't be used when cross compiling"
-    fi
-    continue
   elif [ x"$x" = x"-dumpversion" ]; then
     echo $GCC_VERSION
     exit 0