utils.bbclass/multilib.class: Added misc supporting functions.

utils.bbclass/multilib.class: Added misc supporting functions.

1. Added variable MULTILIB_VARIANTS to store all the instance variants
for multilib extend.

2. Added function all_multilib_tune_values to collect the variable
values for all multilib instance.

3. multilib bbclass handler will save the orignal value of all variables
defined in MULTILIB_SAVE_VARNAME.

(From OE-Core rev: 18bba910e04bff75460f408e4557d4bae21ad592)

Signed-off-by: Lianhao Lu <lianhao.lu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>