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-relocs.patch

@@ -0,0 +1,13 @@
+diff --git a/arch/x86/Makefile b/arch/x86/Makefile
+index fad55160dcb9..a48c8331cbb2 100644
+--- a/arch/x86/Makefile
++++ b/arch/x86/Makefile
+@@ -239,7 +239,7 @@ ifdef CONFIG_RETPOLINE
+ endif
+ 
+ archscripts: scripts_basic
+-	$(Q)$(MAKE) $(build)=arch/x86/tools relocs
++	$(Q)$(MAKE) $(build)=arch/x86/tools
+ 
+ ###
+ # Syscall table generation