[BPF] override getFrameIndexReference to fix stack offset calculation

[bidhan-a]

Summary

The BPF backend did not override getFrameIndexReference() so the base class default impl added StackSize to frame object offsets. This is not needed for BPF where R10 is a fixed frame pointer and stack object offsets are already relative to it

This shifted all stack variable locations by StackSize, resulting in incorrect DWARF debug info.

Example

Before:

from_account:  DW_AT_location (DW_OP_reg10 W10)
to_account:    DW_AT_location (DW_OP_fbreg +8, DW_OP_stack_value)

After:

from_account:  DW_AT_location (DW_OP_fbreg -2040, DW_OP_stack_value)
to_account:    DW_AT_location (DW_OP_fbreg -2040, DW_OP_plus_uconst 0x8, DW_OP_stack_value)