bpf: opcodes, gas: support for signed load V4 instructions

bpf: opcodes, gas: support for signed load V4 instructions

This commit adds the signed load to register (ldxs*) instructions
introduced in the BPF ISA version 4, including opcodes and assembler
tests.

Tested in bpf-unknown-none.

include/ChangeLog:

2023-07-21  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* opcode/bpf.h (enum bpf_insn_id): Add entries for signed load
	instructions.
	(BPF_MODE_SMEM): Define.

opcodes/ChangeLog:

2023-07-21  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* bpf-opc.c (bpf_opcodes): Add entries for LDXS{B,W,H,DW}
	instructions.

gas/ChangeLog:

2023-07-21  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* testsuite/gas/bpf/mem.s: Add signed load instructions.
	* testsuite/gas/bpf/mem-pseudoc.s: Likewise.
	* testsuite/gas/bpf/mem.d: Likewise.
	* testsuite/gas/bpf/mem-pseudoc.d: Likewise.
	* testsuite/gas/bpf/mem-be.d: Likewise.
	* doc/c-bpf.texi (BPF Instructions): Document the signed load
	instructions.