linux-headers: Fix Darwin cross build

Carefully fake cc-version and cc-fullversion to avoid needing a compiler
for the kernel itself to build the headers.

For some reason, doing `make install_headers` twice, first without
INSTALL_HDR_PATH=$out then with, is neccessary to get this to work.

pkgs/os-specific/linux/kernel-headers/no-dynamic-cc-version-check.patch

@@ -0,0 +1,16 @@
+diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
+index 065324a8046f..d09c67194549 100644
+--- a/scripts/Kbuild.include
++++ b/scripts/Kbuild.include
+@@ -216,11 +216,8 @@ cc-disable-warning = $(call try-run-cached,\
+ cc-name = $(call shell-cached,$(CC) -v 2>&1 | grep -q "clang version" && echo clang || echo gcc)
+ 
+ # cc-version
+-cc-version = $(call shell-cached,$(CONFIG_SHELL) $(srctree)/scripts/gcc-version.sh $(CC))
+ 
+ # cc-fullversion
+-cc-fullversion = $(call shell-cached,$(CONFIG_SHELL) \
+-	$(srctree)/scripts/gcc-version.sh -p $(CC))
+ 
+ # cc-ifversion
+ # Usage:  EXTRA_CFLAGS += $(call cc-ifversion, -lt, 0402, -O1)