Symbolize global variables

[zhangzhenghsy]

Currently some global variables in llvm bc files are initialized to Null, it will result in some incorrect results
For example, in file_system_type() function ( https://elixir.bootlin.com/linux/v5.5-rc5/source/fs/filesystems.c#L52 )
file_systems is null in the bc file, and the loop will be skipped and null pointer is returned directly. It will terminate the following execution

Currently, let's symbolize the global variables which equal null first? Maybe we need to symbolize more global variables later

test case, /home/zzhan173/repos/Linux_kernel_UC_KLEE/configs/find_filesystem.json

[ZHYfeng]

d8b7797

[zhangzhenghsy]

Note: now we symbolize all non-constant global variables