libgcc: Add basic support for aarch64-gnu (GNU/Hurd on AArch64)

There is currently no unwinding implementation.

libgcc/ChangeLog:

	* config.host: Recognize aarch64*-*-gnu* hosts.
	* config/aarch64/gnu-unwind.h: New file.
	* config/aarch64/heap-trampoline.c
	(allocate_trampoline_page): Support GNU/Hurd.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>