[NFC] add new function is64Bit for SymbolicFile class

Summary:

since the class 'SymbolicFile ' do not have a is64Bit() API , when we need to check whether a SymbolicFile object is 64bit or not. we need to write a function to do it, it maybe cause duplication code.

Reviewers: James Henderson, Fangrui Song
Differential Revision: https://reviews.llvm.org/D143097