[xen] Allow for platforms that have no Xen support

The Xen headers support only x86 and ARM.  Allow for platforms such as
LoongArch64 to build despite the absence of Xen support by providing
an architecture-specific <bits/xen.h> that simply does:

  #ifndef _BITS_XEN_H
  #define _BITS_XEN_H
  #include <ipxe/nonxen.h>
  #endif /* _BITS_XEN_H */

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/xen/import.pl

@@ -59,6 +59,10 @@ sub try_import_file {
     if ( /^\#include\s+[<\"](\S+)[>\"]/ ) {
       push @dependencies, catfile ( $subdir, $1 );
     }
+    # Patch "Unsupported architecture" line
+    if ( /^\#error\s+"Unsupported\sarchitecture"/ ) {
+      $_ = "#include <bits/xen.h>"
+    }
     # Write out line
     print $outfh "$_\n";
     # Apply FILE_LICENCE() immediately after include guard

src/include/xen/xen.h

@@ -19,7 +19,7 @@ FILE_LICENCE ( MIT );
 #elif defined(__arm__) || defined (__aarch64__)
 #include "arch-arm.h"
 #else
-#error "Unsupported architecture"
+#include <bits/xen.h>
 #endif
 
 #ifndef __ASSEMBLY__