Add memory tagging testcases

Add memory tagging testcases

Add an AArch64-specific test and a more generic memory tagging test that
other architectures can run.

Even though architectures not supporting memory tagging can run the memory
tagging tests, the runtime check will make the tests bail out early, as it
would make no sense to proceed without proper support.

gdb/testsuite/ChangeLog:

YYYY-MM-DD  Luis Machado  <luis.machado@linaro.org>

	* gdb.arch/aarch64-mte.c: New file.
	* gdb.arch/aarch64-mte.exp: New test.
	* gdb.base/memtag.c: New file.
	* gdb.base/memtag.exp: New test.
	* lib/gdb.exp (supports_memtag): New function.