xen: Add Darwin.mk

Xen does not provide a Darwin build configuration useable on Mac OS.
It is possible to build Xen Hypervisor (tools are not supported) with a
toolchain able to produce ELF binaries (GCC or others).

Add config/Darwin.mk to include StdGNU.mk and force
XEN_COMPILE_ARCH=unknown, ensuring Darwin builds always follow
the cross-compile path so compiling on Mac OS is always handled as a
cross compilation case.

Only compiling the hypervisor has been tested !

An example of how to build the hypervisor for arm64 on Mac OS
(tools cannot be build for now) using a compiler from brew:
- brew install aarch64-elf-gcc aarch64-elf-binutils
- make XEN_TARGET_ARCH=arm64 CROSS_COMPILE=aarch64-elf- HOSTCC=clang -C xen

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

config/Darwin.mk

@@ -0,0 +1,7 @@
+# Use GNU tool definitions as the tools we are using are either GNU compatible
+# or we only use features which are supported on Mac OS.
+include $(XEN_ROOT)/config/StdGNU.mk
+
+# Force cross compile on Mac OS: Only hypervisor build is supported, no tools,
+# and in the hypervisor the (plain) SVR4 ABI is in use.
+XEN_COMPILE_ARCH = unknown