[gold] The first patch to address cortex-a53 erratum-843419.

[gold] The first patch to address cortex-a53 erratum-843419.

It implemented scanning the binary and reporting occurrences to users
when '--fix-cortex-a53' is turned on. With this, gold users will be
able to see if or not there are such erratum occurrences in the output
binary. Also included in the CL is reading/recording mapping symbols,
which is needed during scan.

gold/ChangeLog:
    * aarch64.cc (AArch64_insn_utilities): New utility class.
    (AArch64_relobj::Mapping_symbol_position): New struct.
    (AArch64_relobj::Mapping_symbol_info): New typedef.
    (AArch64_relobj::do_count_local_symbols): New function overriding
    parent's implementation.
    (AArch64_relobj::mapping_symbol_info_): New member
    (AArch64_relobj::scan_erratum_843419): New method.
    (Target_aarch64::scan_erratum_843419_span): New method.
    (Target_aarch64::is_erratum_843419_sequence): New method.
    * options.h (fix_cortex_a53): New option.