package/qemu: add BR2_PACKAGE_HOST_QEMU_USER_MODE_ARGS string

package/qemu: add BR2_PACKAGE_HOST_QEMU_USER_MODE_ARGS string

For specific architectures, running qemu in user mode without any additional
options may fail if the host processor does not have the necessary instructions
to properly run qemu in user mode, which results in the following error:
"qemu: uncaught target signal 4 (Illegal instruction) - core dumped"

CoreI7 is one such architecture that has had consistent auto-build failures.

Add a new string in qemu/Config.in.host: BR2_PACKAGE_HOST_QEMU_USER_MODE_ARGS.
The default for the Corei7 architecture is directly from the OpenEmbedded
project found in meta/conf/machine/include/x86/tune-corei7.inc:
"-cpu Nehalem,check=false." Other architectures may be added to this string at
a later date if other failures occure.

Signed-off-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>