[BOLT][AArch64] Handle data at the beginning of a function when disassembling and building CFG.

This patch adds getFirstInstructionOffset method for BinaryFunction
which is used to properly handle cases where data is at zero offset in
a function. The main change is that we add basic block at first
instruction offset when disassembling, which prevents assertion
failures in buildCFG.

Reviewed By: yota9, rafauler

Differential Revision: https://reviews.llvm.org/D127111