[gdb/aarch64] Use safer memory read routines

  PR tdep/28796

  As reported, we are using some memory read routines that don't handle read
  errors gracefully. Convert those to use the safe_* versions if available.

  This allows the code to handle those read errors in a more sensible way.

  Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28796