meson: drop libdl, threads, and librt dependencies

Our baseline glibc is 2.34, which merged libdl, libpthread (the
dependency('threads') target), and librt into libc. Empty .so/.a stubs
remain for backward compatibility with old binaries, but new builds
resolve dl_*, pthread_*, mq_*, timer_*, etc. directly from libc.
On musl the same libraries are likewise empty stubs.

Drop the libdl, threads, and librt entries from every meson.build, and
remove the now-stale 'Libs.private: -lrt -pthread' from libudev.pc.in
since both flags resolve to empty link-time stubs on glibc 2.34+ and
musl.

Verified with readelf -d that libsystemd.so, libudev.so, and systemd no
longer carry DT_NEEDED entries for libdl/libpthread/librt.

src/core/meson.build

@@ -148,13 +148,10 @@ libcore_static = static_library(
         dependencies : [libaudit_cflags,
                         libbpf_cflags,
                         libcryptsetup_cflags,
-                        libdl,
                         libm,
                         libmount_cflags,
-                        librt,
                         libseccomp_cflags,
                         libselinux_cflags,
-                        threads,
                         userspace],
         build_by_default : false)