The _start() function attempts to call main() which is a
problem when trying to build a shared library with --no-undefined:
$ echo > t.c
$ gcc -shared -Wl,--no-undefined -o t.so t.c
/usr/local/lib/gcc/i686-pc-serenity/10.2.0/../../../../i686-pc-serenity/bin/ld:
/usr/lib/crt0_shared.o: in function `_start':
./Build/i686/../../Userland/Libraries/LibC/crt0_shared.cpp:56: undefined reference to `main'
collect2: error: ld returned 1 exit status
Also, unless explicitly requested by the user shared libraries
should not have an entry point (e.g. _start) at all.
