libc,shared: detect newer library symbols at runtime

For libc syscall wrappers (pidfd_open, fsopen, openat2, etc.) we previously
gated the calls behind build-time HAVE_* checks. Replace these with shim
functions in src/libc/ that fall back to the raw syscall at runtime when the
loaded glibc lacks the symbol. The infrastructure lives in src/libc/libc-shim.h:
DEFINE_SYSCALL_SHIM falls back to a direct syscall, DEFINE_LIBC_SHIM returns
ENOSYS (for posix_spawn-family helpers that have no corresponding syscall), and
DEFINE_LIBC_ERRNO_SHIM sets errno=ENOSYS and returns -1 (for read/write-style
helpers). The weak reference to the libc symbol is bound via __asm__(\"name\")
rename so the bare libc identifier never appears as a C token — this avoids
both #undef boilerplate against override-header redirects and the resulting
-Wredundant-decls warning. Drop the corresponding cc.has_function() loop from
meson.build.

For optional libraries (libcryptsetup, libdw, libarchive), drop the per-symbol
HAVE_* checks. Always declare the prototypes, suppressing the redundant-decl
warnings via DISABLE_WARNING_REDUNDANT_DECLS and NOLINT, and resolve the symbols
after the main dlopen via a new DLSYM_OPTIONAL() helper that only assigns on
success. libcryptsetup's crypt_set_keyring_to_link / crypt_token_set_external_path
and libarchive's *_is_set wrappers use fallback functions as their pointer
initializers (returning -ENOSYS and 0 respectively), so call sites can invoke
the symbol unconditionally and just check for -ENOSYS where the \"not supported\"
distinction matters.

The same shim treatment applies to pidfd_spawn / posix_spawnattr_setcgroup_np
(src/libc/spawn.c) and epoll_pwait2 (src/libc/epoll.c), with corresponding
override headers in src/include/override/spawn.h and
src/include/override/sys/epoll.h. posix_spawn_wrapper() in process-util.c and
epoll_wait_usec() in sd-event.c now detect ENOSYS in the return value instead
of checking the function pointer, falling back to plain posix_spawn() and
epoll_wait() respectively. coredump-config and coredump-submit get a
dlopen_dw_has_dwfl_set_sysroot() helper. The kexec arch gate now uses
defined(__NR_kexec_file_load) directly; pidfd.h uses __has_include_next() to
decide whether to pull in glibc's header.

This lets binaries built against newer glibc / libcryptsetup / libdw /
libarchive headers still load and run on older targets where these symbols are
absent.

src/coredump/coredump-config.c

@@ -2,6 +2,7 @@
 
 #include "conf-parser.h"
 #include "coredump-config.h"
+#include "elf-util.h"
 #include "format-util.h"
 #include "journal-def.h"
 #include "log.h"
@@ -45,12 +46,10 @@ int coredump_parse_config(CoredumpConfig *config) {
                 config->journal_size_max = JOURNAL_SIZE_MAX;
         }
 
-#if !HAVE_DWFL_SET_SYSROOT
-        if (config->enter_namespace) {
+        if (config->enter_namespace && !dlopen_dw_has_dwfl_set_sysroot()) {
                 log_warning("EnterNamespace= is enabled but libdw does not support dwfl_set_sysroot(), disabling.");
                 config->enter_namespace = false;
         }
-#endif
 
         log_debug("Selected storage '%s'.", coredump_storage_to_string(config->storage));
         log_debug("Selected compression %s.", yes_no(config->compress));

src/coredump/coredump-submit.c

@@ -468,7 +468,9 @@ static int maybe_remove_external_coredump(
 }
 
 static int acquire_pid_mount_tree_fd(const CoredumpConfig *config, CoredumpContext *context) {
-#if HAVE_DWFL_SET_SYSROOT
+        if (!dlopen_dw_has_dwfl_set_sysroot())
+                return log_debug_errno(SYNTHETIC_ERRNO(EOPNOTSUPP), "dwfl_set_sysroot() is not supported.");
+
         _cleanup_close_ int mntns_fd = -EBADF, root_fd = -EBADF, fd = -EBADF;
         _cleanup_close_pair_ int pair[2] = EBADF_PAIR;
         int r;
@@ -533,10 +535,6 @@ static int acquire_pid_mount_tree_fd(const CoredumpConfig *config, CoredumpConte
 
         context->mount_tree_fd = TAKE_FD(fd);
         return 0;
-#else
-        /* Don't bother preparing environment if we can't pass it to libdwfl. */
-        return log_debug_errno(SYNTHETIC_ERRNO(EOPNOTSUPP), "dwfl_set_sysroot() is not supported.");
-#endif
 }
 
 static int attach_mount_tree(const CoredumpConfig *config, CoredumpContext *context) {