aarch64: detect atomic sequences like other ll/sc architectures

gdb/Changelog:

        * aarch64-tdep.c (aarch64_software_single_step): New function.
        (aarch64_gdbarch_init): Handle single stepping of atomic sequences
        with aarch64_software_single_step.

gdb/testsuite/ChangeLog:

        * gdb.arch/aarch64-atomic-inst.c: New file.
        * gdb.arch/aarch64-atomic-inst.exp: New file.