bpf: support `naked' function attributes in BPF targets

The kernel selftests and other BPF programs make extensive use of the
`naked' function attribute with bodies written using basic inline
assembly.  This patch adds support for the attribute to
bpf-unkonwn-none, makes it to inhibit warnings due to lack of explicit
`return' statement, and updates documentation and testsuite
accordingly.

Tested in x86_64-linux-gnu host and bpf-unknown-none target.

gcc/ChangeLog

	PR target/111046
	* config/bpf/bpf.cc (bpf_attribute_table): Add entry for the
	`naked' function attribute.
	(bpf_warn_func_return): New function.
	(TARGET_WARN_FUNC_RETURN): Define.
	(bpf_expand_prologue): Add preventive comment.
	(bpf_expand_epilogue): Likewise.
	* doc/extend.texi (BPF Function Attributes): Document the `naked'
	function attribute.

gcc/testsuite/ChangeLog

	* gcc.target/bpf/naked-1.c: New test.