[aarch64] Refactor common errata work-around specs

[aarch64] Refactor common errata work-around specs

With the addition of the AArch64 NetBSD support code we now have four
ports all using and defining the same errata work-around headers.
That's silly and long-term becomes a maintenance burden.

This patch factors all that code into a single header to eliminate all
the duplication.

	* config/aarch64/aarch64-errata.h: New file.
	* config/aarch64/aarch64-elf-raw.h (CA53_ERR_835769_SPEC): Delete.
	(CA53_ERR_843419_SPEC): Delete.
	(LINK_SPEC): Use AARCH64_ERRATA_LINK_SPEC instead of above.
	* config/aarch64/aarch64-linux.h: Likewise.
	* config/aarch64/aarch64-netbsd.h: Likewise.
	* config/aarch64/aarch64-freebsd.h: Likewise.

From-SVN: r272515