bpf: make use of the bswap{16,32,64} V4 BPF instruction

bpf: make use of the bswap{16,32,64} V4 BPF instruction

This patch makes the BPF backend to use the new V4 bswap{16,32,64}
instructions in order to implement the __builtin_bswap{16,32,64}
built-ins.  It also adds support for -mcpu=v4 and -m[no]bswap
command-line options.  Tests and doc updates are includes.

Tested in bpf-unknown-none.

gcc/ChangeLog

	PR target/110786
	* config/bpf/bpf.opt (mcpu): Add ISA_V4 and make it the default.
	(mbswap): New option.
	* config/bpf/bpf-opts.h (enum bpf_isa_version): New value ISA_V4.
	* config/bpf/bpf.cc (bpf_option_override): Set bpf_has_bswap.
	* config/bpf/bpf.md: Use bswap instructions if available for
	bswap* insn, and fix constraint.
	* doc/invoke.texi (eBPF Options): Document -mcpu=v4 and -mbswap.

gcc/testsuite/ChangeLog

	PR target/110786
	* gcc.target/bpf/bswap-1.c: Pass -mcpu=v3 to build test.
	* gcc.target/bpf/bswap-2.c: New test.