Say we also implement librt.so.1.

Say we also implement librt.so.1. This is required, for example, by the
Boost libraries (e.g., libboost_system-mt.so.1.50.0). The librt library
isn't actually a separate library on modern Linux - rather all its
traditional functions are now in glibc, and "librt" is merely a filter
on glibc. So no reason not to say we support librt too.

Not to mention that we already implement a bunch of functions that
traditionally resided in librt (nanosleep, sched_yield, sem_*, etc.

core/elf.cc

@@ -650,6 +650,7 @@ program::program(::filesystem& fs, void* addr)
     set_object("ld-linux-x86-64.so.2", _core.get());
     set_object("libpthread.so.0", _core.get());
     set_object("libdl.so.2", _core.get());
+    set_object("librt.so.1", _core.get());
 }
 
 void program::set_search_path(std::initializer_list<std::string> path)