autoconf: remove i386 windows related logic

This commit removes legacy i386 windows logic in autoconf scripts.

m4/fp_leading_underscore.m4

@@ -24,7 +24,6 @@ case $TargetPlatform in
         i386-*2\.@<:@0-9@:>@ | i386-*3\.@<:@0-3@:>@ ) fptools_cv_leading_underscore=yes ;;
         *) fptools_cv_leading_underscore=no ;;
       esac ;;
-    i386-unknown-mingw32) fptools_cv_leading_underscore=yes;;
     x86_64-unknown-mingw32) fptools_cv_leading_underscore=no;;
     *) AC_RUN_IFELSE([AC_LANG_SOURCE([[#if defined(HAVE_NLIST_H)
 #include <nlist.h>

m4/fp_setup_windows_toolchain.m4

 # Download and install the windows toolchain
 AC_DEFUN([FP_INSTALL_WINDOWS_TOOLCHAIN],[
     # Find the mingw-w64 archive file to extract.
-    if test "$HostArch" = "i386"
+    if test "$HostArch" = "x86_64"
     then
-        mingw_arch="i686"
-        tarball_dest_dir="mingw-w64/i686"
-        tarball_mingw_dir="clang32"
-    else
         mingw_arch="x86_64"
         tarball_dest_dir="mingw-w64/x86_64"
         tarball_mingw_dir="clang64"

m4/fptools_set_c_ld_flags.m4

@@ -33,9 +33,6 @@ AC_DEFUN([FPTOOLS_SET_C_LD_FLAGS],
     esac
 
     case $$1 in
-    i386-unknown-mingw32)
-        $2="$$2 -march=i686"
-        ;;
     i386-portbld-freebsd*)
         $2="$$2 -march=i686"
         ;;

m4/ghc_select_file_extensions.m4

@@ -5,10 +5,10 @@ AC_DEFUN([GHC_SELECT_FILE_EXTENSIONS],
     case $1 in
     *-unknown-cygwin32)
         AC_MSG_WARN([GHC does not support the Cygwin target at the moment])
-        AC_MSG_WARN([I'm assuming you wanted to build for i386-unknown-mingw32])
+        AC_MSG_WARN([I'm assuming you wanted to build for x86_64-w64-mingw32])
         exit 1
         ;;
-    # examples: i386-unknown-mingw32, i686-w64-mingw32, x86_64-w64-mingw32
+    # examples: x86_64-w64-mingw32
     *-mingw32)
         windows=YES
         $2='.exe'