configure.ac: fix NCG support in --target=

Before this change attempt to build a crosscompiler
on registerised platform (--host=x86_64-pc-linux-gnu)
targeting UNREG platform failed:
    $ ./configure --target=ia64-unknown-linux-gnu

  utils/genapply/../../includes/stg/MachRegs.h:608:2: error:
     #error Cannot find platform to give register info for

The change is to check --target= for NCG availability,
not --host=.
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

configure.ac

@@ -241,7 +241,7 @@ AC_SUBST(SOLARIS_BROKEN_SHLD)
 
 dnl ** Do an unregisterised build?
 dnl --------------------------------------------------------------
-case "$HostArch" in
+case "$TargetArch" in
     i386|x86_64|powerpc|powerpc64|powerpc64le|arm)
         UnregisterisedDefault=NO
         ;;