src/basic: generate missing syscall headers programatically

Getting the numbers right for all architectures has proven to be a
constant chore. Let's autogenerate the header from the tables that
were imported in one of the previous commits.

Fixes #18074. (Hopefully. I cannot verify this on all architectures.)

To update the lists, or to update the header after template changes:
ninja -C build update-syscall-tables update-syscall-header

Note: the generated file is saved in git. Initially I wanted to only
store the tables in git, and generate the header during each build.
Generation is quick enough, but the header is used in many many
places (wherever missing_syscall.h is included, directly or indirectly),
which means that we would need to declare the dependency in meson, so
the header would be generated early enough. This turned out to be very
noisy. Storing the generated header in version control avoids the hassle.

src/basic/meson.build

@@ -262,6 +262,9 @@ missing_audit_h = files('missing_audit.h')
 missing_capability_h = files('missing_capability.h')
 missing_socket_h = files('missing_socket.h')
 
+missing_syscall_def_h = files('missing_syscall_def.h')
+basic_sources += missing_syscall_def_h
+
 generate_af_list = find_program('generate-af-list.sh')
 af_list_txt = custom_target(
         'af-list.txt',
@@ -330,32 +333,6 @@ foreach item : [['af',     af_list_txt,     'af',         ''],
 endforeach
 
 basic_sources += generated_gperf_headers
-basic_gcrypt_sources = files(
-        'gcrypt-util.c',
-        'gcrypt-util.h')
-
-libbasic = static_library(
-        'basic',
-        basic_sources,
-        include_directories : includes,
-        dependencies : [versiondep,
-                        threads,
-                        libcap,
-                        libseccomp,
-                        libselinux,
-                        libm,
-                        libdl],
-        c_args : ['-fvisibility=default'],
-        install : false)
-
-# A convenience library that is separate from libbasic to avoid
-# unnecessary linking to libgcrypt.
-libbasic_gcrypt = static_library(
-        'basic-gcrypt',
-        basic_gcrypt_sources,
-        include_directories : includes,
-        dependencies : [libgcrypt],
-        c_args : ['-fvisibility=default'])
 
 ############################################################
 
@@ -382,3 +359,47 @@ run_target(
         command : [syscall_table_update_sh, meson.current_source_dir()] + arch_list)
 
 syscall_names_text = files('syscall-names.text')
+
+syscall_lists = []
+foreach arch: arch_list
+        syscall_lists += files('syscalls-@0@'.format(arch))
+endforeach
+
+missing_syscalls_py = find_program('missing_syscalls.py')
+
+run_target(
+        'update-syscall-header',
+        command : [missing_syscalls_py,
+                   missing_syscall_def_h,
+                   syscall_lists])
+
+############################################################
+
+libbasic = static_library(
+        'basic',
+        basic_sources,
+        include_directories : includes,
+        dependencies : [versiondep,
+                        threads,
+                        libcap,
+                        libseccomp,
+                        libselinux,
+                        libm,
+                        libdl],
+        c_args : ['-fvisibility=default'],
+        install : false)
+
+############################################################
+
+basic_gcrypt_sources = files(
+        'gcrypt-util.c',
+        'gcrypt-util.h')
+
+# A convenience library that is separate from libbasic to avoid
+# unnecessary linking to libgcrypt.
+libbasic_gcrypt = static_library(
+        'basic-gcrypt',
+        basic_gcrypt_sources,
+        include_directories : includes,
+        dependencies : [libgcrypt],
+        c_args : ['-fvisibility=default'])