treewide: drop librt and libpthread packages

treewide: drop librt and libpthread packages

Since OpenWrt's main libc library, musl, does not provide separate shared
object files for libpthread and librt, the existing binary packages for
them are empty placeholders which provide no runtime functionality and
frequently cause confusion among users who attempt to build software
linking -lrt or -lpthread on target.

To clean this situation up somewhat and to simplify binary package
dependecies for all of the potential musl, glibc and uclibc cases, drop
those packages and move libpthread.so as well as librt.so into the main
libc package for those libc implementations that happen ship them as
extra shared libraries.

Also let the abstract libc package provide virtual libpthread and librt
packages in order to ease migration for packages that still explicitly
depend on them.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>