testsuite: Add filter for target socket support

The new analyzer tests for sockets are failing on embedded targets.
The newlib and avr-libc C libraries do not support sockets.

Testing done:
  - No changes in gcc.sum for x86_64-pc-linux-gnu, with or without this
    patch.
  - Filtered cases are now UNSUPPORTED instead of failing on AVR and PRU
    backends.

gcc/ChangeLog:

	* doc/sourcebuild.texi (sockets): Document new check.

gcc/testsuite/ChangeLog:

	* gcc.dg/analyzer/fd-accept.c: Require sockets.
	* gcc.dg/analyzer/fd-bind.c: Ditto.
	* gcc.dg/analyzer/fd-connect.c: Ditto.
	* gcc.dg/analyzer/fd-datagram-socket.c: Ditto.
	* gcc.dg/analyzer/fd-glibc-byte-stream-connection-server.c:
	Ditto.
	* gcc.dg/analyzer/fd-glibc-byte-stream-socket.c: Ditto.
	* gcc.dg/analyzer/fd-glibc-datagram-client.c: Ditto.
	* gcc.dg/analyzer/fd-glibc-datagram-socket.c: Ditto.
	* gcc.dg/analyzer/fd-listen.c: Ditto.
	* gcc.dg/analyzer/fd-manpage-getaddrinfo-client.c: Ditto.
	* gcc.dg/analyzer/fd-mappage-getaddrinfo-server.c: Ditto.
	* gcc.dg/analyzer/fd-socket-meaning.c: Ditto.
	* gcc.dg/analyzer/fd-socket-misuse.c: Ditto.
	* gcc.dg/analyzer/fd-stream-socket-active-open.c: Ditto.
	* gcc.dg/analyzer/fd-stream-socket-passive-open.c: Ditto.
	* gcc.dg/analyzer/fd-stream-socket.c: Ditto.
	* gcc.dg/analyzer/fd-symbolic-socket.c: Ditto.
	* lib/target-supports.exp (check_effective_target_sockets): New
	check.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>