Support grub-emu on x32 (ILP32 but with x86-64 instruction set)

* configure.ac: Remove -m64 from checks for -mcmodel=large and
-mno-red-zone.  These are always either unnecessary (x86_64-emu) or
already in TARGET_CFLAGS at this point, and they produce incorrect
results when building for x32.
* grub-core/kern/x86_64/dl.c (grub_arch_dl_relocate_symbols): Cast
pointers to Elf64_Xword via grub_addr_t, in order to work on x32.
* include/grub/x86_64/types.h (GRUB_TARGET_SIZEOF_VOID_P,
GRUB_TARGET_SIZEOF_LONG): Define to 4 on x32.

configure.ac

@@ -862,7 +862,7 @@ LDFLAGS="$TARGET_LDFLAGS"
 if test "$target_cpu" = x86_64 || test "$target_cpu-$platform" = sparc64-emu ; then
   # Use large model to support 4G memory
   AC_CACHE_CHECK([whether option -mcmodel=large works], grub_cv_cc_mcmodel, [
-    CFLAGS="$TARGET_CFLAGS -m64 -mcmodel=large"
+    CFLAGS="$TARGET_CFLAGS -mcmodel=large"
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[]])],
 		      [grub_cv_cc_mcmodel=yes],
 		      [grub_cv_cc_mcmodel=no])
@@ -877,7 +877,7 @@ fi
 if test "$target_cpu"-"$platform" = x86_64-efi; then
   # EFI writes to stack below %rsp, we must not use the red zone
   AC_CACHE_CHECK([whether option -mno-red-zone works], grub_cv_cc_no_red_zone, [
-    CFLAGS="$TARGET_CFLAGS -m64 -mno-red-zone"
+    CFLAGS="$TARGET_CFLAGS -mno-red-zone"
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[]])],
 		      [grub_cv_cc_no_red_zone=yes],
 		      [grub_cv_cc_no_red_zone=no])