Add gdbserver memory tagging support
    
    Adds the AArch64-specific memory tagging support (MTE) by implementing the
    required hooks and checks.
    
    gdbserver/ChangeLog:
    
    YYYY-MM-DD  Luis Machado  <luis.machado@linaro.org>
    
    	* Makefile.in (SFILES): Add /../gdb/nat/aarch64-mte-linux-ptrace.c.
    	* configure.srv (aarch64*-*-linux*): Add arch/aarch64-linux.o and
    	nat/aarch64-mte-linux-ptrace.o.
    	* linux-aarch64-low.cc: Include nat/aarch64-mte-linux-ptrace.h.
    	(class aarch64_target) <supports_memory_tagging>
    	<fetch_memtags, store_memtags>: New method overrides.
    	(aarch64_target::supports_memory_tagging)
    	(aarch64_target::fetch_memtags)
    	(aarch64_target::store_memtags): New methods.