LibC: Add stub forwarders to LibRegex C API

The POSIX C regex functions are expected to live in the C standard
library, but Serenity split off its regex library into LibRegex. Make a
compromise by implementing stub forwarders for the C regex library that
load libregex.so and call the real implementation.

This is needed for ports that expect these C functions to be available
inside the standard C library without introducing a strong coupling
between LibC and LibDl or LibRegex. The non-standard Serenity C++ regex
API still lives inside LibRegex as before.

Userland/Libraries/LibC/CMakeLists.txt

@@ -26,6 +26,7 @@ set(LIBC_SOURCES
     pty.cpp
     pwd.cpp
     qsort.cpp
+    regex.cpp
     scanf.cpp
     sched.cpp
     serenity.cpp