basic/missing-syscalls: only emit one warning about missing numbers

The ifdef pattern is the same for all syscalls, so most of the time, if one is
not defined, all others will too. So let's reduce the noise a bit and emit one
warning in case the support for the architecture is fully missing. (Current
template was copied over from before when we added numbers for each syscall by
hand and stopped making sense when we started generating the header from a
table that is expected to have all syscall numbers.)

src/basic/missing_syscalls.py

@@ -37,8 +37,12 @@ def parse_syscall_tables(filenames):
     return {filename.split('-')[-1][:-4]: parse_syscall_table(filename)
             for filename in filenames}
 
-DEF_TEMPLATE = '''
+DEF_TEMPLATE_A = '''\
+
 #ifndef __IGNORE_{syscall}
+'''
+
+DEF_TEMPLATE_B = '''\
 #  if defined(__aarch64__)
 #    define systemd_NR_{syscall} {nr_arm64}
 #  elif defined(__alpha__)
@@ -75,9 +79,12 @@ DEF_TEMPLATE = '''
 #    else
 #      define systemd_NR_{syscall} {nr_x86_64}
 #    endif
-#  else
-#    warning "{syscall}() syscall number is unknown for your architecture"
+#  elif !defined(missing_arch_template)
+%s
 #  endif
+'''
+
+DEF_TEMPLATE_C = '''\
 
 /* may be an (invalid) negative number due to libseccomp, see PR 13319 */
 #  if defined __NR_{syscall} && __NR_{syscall} >= 0
@@ -92,20 +99,33 @@ assert_cc(__NR_{syscall} == systemd_NR_{syscall});
 #      define __NR_{syscall} systemd_NR_{syscall}
 #    endif
 #  endif
-#endif
-'''
+#endif'''
+
+DEF_TEMPLATE = (DEF_TEMPLATE_A +
+                DEF_TEMPLATE_B % '#    warning "{syscall}() syscall number is unknown for your architecture"' +
+                DEF_TEMPLATE_C)
+
+ARCH_CHECK = '''\
+/* Note: if this code looks strange, this is because it is derived from the same
+ * template as the per-syscall blocks below. */
+''' + '\n'.join(line for line in DEF_TEMPLATE_B.splitlines()
+                       if ' define ' not in line) % '''\
+#    warning "Current architecture is missing from the template"
+#    define missing_arch_template 1'''
 
 def print_syscall_def(syscall, tables, out):
     mappings = {f'nr_{arch}':t.get(syscall, -1)
                 for arch, t in tables.items()}
     print(DEF_TEMPLATE.format(syscall=syscall, **mappings),
-          file=out, end='')
+          file=out)
 
 def print_syscall_defs(syscalls, tables, out):
     print('''\
 /* SPDX-License-Identifier: LGPL-2.1-or-later
  * This file is generated. Do not edit! */
-''' , file=out, end='')
+''',
+          file=out)
+    print(ARCH_CHECK, file=out)
     for syscall in syscalls:
         print_syscall_def(syscall, tables, out)