populate_sdk_base.bbclass: fix SDKTARGETSYSROOT value
Currently, SDKTARGETSYSROOT points to PACKAGE_ARCH which, sometimes, can
be set to MACHINE_ARCH. When this happens, the default target sysroot
passed to the cross-canadian toolchain, which points to TUNE_PKGARCH,
will be different from the directory where the target sysroot has been
deployed.
In order to fix this, use REAL_MULTIMACH_TARGET_SYS variable instead of
MULTIMACH_TARGET_SYS.
