multilib_global.bbclass: fix PREFERRED_VERSION for cross-canadian

multilib_global.bbclass: fix PREFERRED_VERSION for cross-canadian

Fixed:
When both gcc 4.9 and 5.1 are in the layer, and set these in local.conf:
GCCVERSION = "4.9%"
MACHINE = "qemux86-64"
require conf/multilib.conf
MULTILIBS = "multilib:lib32"
DEFAULTTUNE_virtclass-multilib-lib32 = "x86"

$ bitbake gcc-cross-canadian-i586

Then gcc 5.1 would be built rather than 4.9, it is because
multilib_virtclass_handler() doesn't reset PN such as gcc-cross-canadian
to lib32-gcc-cross-canadian, but preferred_ml_updates() rename
PREFERRED_VERSION_gcc-cross-canadian to
PREFERRED_VERSION_lib32-gcc-cross-canadian, so it can't get the required
value PREFERRED_VERSION_gcc-cross-canadian, and then gcc 5.1 would be
used, don't reset it in preferred_ml_updates() will fix the problem.

[YOCTO #7950]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>