Move -lsocket check to common.m4

A user pointed out that the -lsocket check in gdb should also apply to
gdbserver -- otherwise it can't find the Solaris socketpair.  This
patch makes the change.  It also removes a couple of redundant
function checks from gdb's configure.ac.

This was tested by the person who reported the bug.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30927
Approved-By: Pedro Alves <pedro@palves.net>