[libc++abi] NFC: adding a new parameter base to functions for calculating address...

[libc++abi] NFC: adding a new parameter base to functions for calculating addresses with relative encodings

    Summary:
    This NFC patch adds a new parameter base to functions invoked by scan_eh_tab() for calculating the address of the encoding with a relative value. base defaults to 0. This is in preparation for the AIX implementation which uses the DW_EH_PE_datarel encoding.

    Reviewed by: MaskRay, sfertile, compnerd, libc++abi

    Differential Revision: https://reviews.llvm.org/D101545