AArch64: Add MTE CPU feature check support

AArch64: Add MTE CPU feature check support

This patch is a preparation for the next patches implementing MTE. It just adds
a HWCAP2 constant for MTE, creates a new generic arch/aarch64-mte-linux.h file
and includes that file in the source files that will use it.

gdb/ChangeLog:

2021-03-24  Luis Machado  <luis.machado@linaro.org>

	* Makefile.in (HFILES_NO_SRCDIR): Add arch/aarch64-mte-linux.h.
	* aarch64-linux-nat.c: Include arch/aarch64-mte-linux.h.
	* aarch64-linux-tdep.c: Likewise
	* arch/aarch64-mte-linux.h: New file.

gdbserver/ChangeLog:

2021-03-24  Luis Machado  <luis.machado@linaro.org>

	* linux-aarch64-low.cc: Include arch/aarch64-mte-linux.h.