build: Add initial ARM64 target.

It will probably be just stubs for the significant future, but,
here it is anyway.

Regarding the naming: Yes, the official name is "aarch64." However,
Linux, FreeBSD, and Zircon all call it "arm64", and so we will do the same.

I've configured it initially to be a Clang-only port, making no
changes to GCC buildtools whatsoever here. We'll see if that sticks,
however.

headers/config/HaikuConfig.h

@@ -56,6 +56,11 @@
 #	define __HAIKU_ARCH_ABI				"armeb"
 #	define __HAIKU_ARCH_ARM				1
 #	define __HAIKU_BIG_ENDIAN			1
+#elif defined(__arm64__)
+#	define __HAIKU_ARCH					arm64
+#	define __HAIKU_ARCH_ABI				"arm64"
+#	define __HAIKU_ARCH_ARM64			1
+#	define __HAIKU_ARCH_BITS			64
 #else
 #	error Unsupported architecture!
 #endif