package/qemu: separate user and system arch on powerpc64le

package/qemu: separate user and system arch on powerpc64le

QEMU provides a single system emulator that supports both powerpc64
and powerpc64le with a target called 'ppc64-softmmu', but it provides
a different usermode emulator for each one (with targets
'ppc64le-linux-user' and 'ppc64-linux-user').

Due to this asymmetry it is not possible to support both cases with
the single arch value used in the package file. This patch introduces
an additional value into the package configuration,
HOST_QEMU_SYS_ARCH, so that both cases can be supported.

Fixes commit d2ff457e88ff4106f97e29516d467978827d30c2
and autobuilder failture
http://autobuild.buildroot.net/results/a2d63e21c3e82c36f4a975e90ed56faba18e97a5

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>