[PATCH v4] xen: Add Darwin.mk for GNU toolchains

[Bertrand Marquis <bertrand.marquis@xxxxxxx>]

Xen does not provide a Darwin build configuration for selecting
GNU tool definitions. On macOS, the tools we use are either GNU
compatible or we only rely on features supported by Mac OS, so
using the GNU tool definitions is appropriate.

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

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

Signed-off-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
---
Changes since v3:
- set XEN_COMPILE_ARCH to unknown instead of Darwin
- list binutils as a dependency to install in brew in commit message

Changes since v2:
- Subject was "xen: Add macOS hypervisor build configuration"
- Update Darwin.mk comments to more accurate versions (Jan)
- Remove the build-on-macos help as we have no dependency on anything
  coming from brew anymore and the toolchain can be retrieved by lots of
  other solutions than brew on mac os. Switch to a simple doc in the
  commit message instead
---
 config/Darwin.mk | 7 +++++++
 1 file changed, 7 insertions(+)
 create mode 100644 config/Darwin.mk

diff --git a/config/Darwin.mk b/config/Darwin.mk
new file mode 100644
index 000000000000..176b27eac676
--- /dev/null
+++ b/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
+
+# Xen uses Linux'es ABI so we are cross compiling on Mac OS.
+# Force COMPILE_ARCH to a fake value to make sure it is always the case.
+XEN_COMPILE_ARCH = unknown
-- 
2.52.0