bpf: Add atomic compare-and-exchange instructions

bpf: Add atomic compare-and-exchange instructions

This patch adds the two remaining BPF v3 atomic instructions:
- BPF_INSN_ACMP{,32}: atomic compare-and-swap
- BPF_INSN_AXCHG{,32}: atomic (non-conditional) exchange

Tests and documentation are also updated.

gas/
	* doc/c-bpf.texi (BPF Instructions): Document atomic exchange and
	atomic compare-and-swap instructions.
	* testsuite/gas/bpf/atomic.s: Test ACMP, ACMP32, AXCHG, AXCGH32
	instructions.
	* testsuite/gas/bpf/atomic.d: Likewise.
	* testsuite/gas/bpf/atomic-be.d: Likewise.
	* testsuite/gas/bpf/atomic-pseudoc.s: Likewise.
	* testsuite/gas/bpf/atomic-pseudoc.d: Likewise.
	* testsuite/gas/bpf/atomic-be-pseudoc.d: Likewise.

include/
	* opcode/bpf.h (BPF_IMM32_ACMP): Fix typo.
	(enum bpf_insn_id): New entries for BPF_INSN_ACMP{,32} and
	BPF_INSN_AXCHG{,32}.

opcodes/
	* bpf-opc.c (bpf_opcodes): Add entries for ACMP{,32} and
	AXCHG{,32} instructions.