Initial support for AArch64 running NetBSD.

Matthew Green <mrg@eterna.com.au>
Maya Rashish <coypu@sdf.org>

gcc:
	* config.gcc (aarch64*-*-netbsd*): New target.
	* config/aarch64/aarch64-netbsd.h: New file.
	* config/aarch64/t-aarch64-netbsd: Likewise.

libgcc:
	* config.host (aarch64*-*-netbsd*): New case.

Co-Authored-By: Maya Rashish <coypu@sdf.org>

From-SVN: r272387