Implement KABI global variable comparison.

[lenticularis39]

KABI lists contain two types of symbols - function and global variables. Currently values that aren't function are skipped. To implement global variable comparison for each global variable a list containing all functions that use it must be built using functions from llvmcpy and stored in functions.yaml as a group containing the global variable name (similarly to how sysctl comparison is handled). Then the compare function must be modified to be able to handle the new kind of group.