grub-1.99: use gcc-cross for building the target binaries

grub-1.99: use gcc-cross for building the target binaries

It was using distro gcc to build binaries for target. This got detected
on fedora 17 alpha, on which it hit an gcc-4.7 issue.

This Fixes Bug: [Yocto #2291]

More information in the patch header

Thanks to Saul Wold for suggesting to remove the --target parameter from
EXTRA_OECONF var.

Signed-off-by: Nitin A Kamble <nitin.a.kamble@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>