config: Fix host -rdynamic detection for build != host != target

[Merge from GCC commit 4d9bc81a5d8d884dee7a7781fa4c1577a6c9681a.]

The GCC_ENABLE_PLUGINS configure logic for detecting whether -rdynamic
is necessary and supported uses an appropriate objdump for $host
binaries (running on $build) in cases where $host is $build or
$target.

However, it is missing such logic in the case where $host is neither
$build nor $target, resulting in the compilers not being linked with
-rdynamic and plugins not being usable with such a compiler.  In fact
$ac_cv_prog_OBJDUMP, as used when $build = $host, is always an objdump
for $host binaries that runs on $build; that is, it's appropriate to
use in this case as well.

Tested in such a configuration that it does result in cc1 being linked
with -rdynamic as expected.  Also bootstrapped with no regressions for
x86_64-pc-linux-gnu.

config/
	* gcc-plugin.m4 (GCC_ENABLE_PLUGINS): Use
	export_sym_check="$ac_cv_prog_OBJDUMP -T" also when host is not
	build or target.

config/gcc-plugin.m4

@@ -49,7 +49,7 @@ AC_DEFUN([GCC_ENABLE_PLUGINS],
        elif test x$host = x$target; then
 	 export_sym_check="$gcc_cv_objdump -T"
        else
-	 export_sym_check=
+	 export_sym_check="$ac_cv_prog_OBJDUMP -T"
        fi
      ;;
    esac