bpf: gas: support relaxation of V4 jump instructions

bpf: gas: support relaxation of V4 jump instructions

The BPF jump-always instruction (JA), like all other jump instructions
in the ISA, get a signed 16-bit displacement target argument denoted
in number of 64-bit words minus one.  This can sometimes be overflown.

The BPF V4 ISA thus introduced support for a jump-always
instruction (JAL) that gets a signed 32-bit displacement instead.

This patch makes the BPF assembler to perform the following
relaxations when the disp16 field gets overflown, unless the option
-mno-relax is specified:

  JA disp16  -> JAL disp32
  Jxx disp16 -> Jxx +1; JA +1; JAL disp32

Documentation and tests added.
Tested in bpf-unknown-none.

gas/ChangeLog:

2023-07-28  Jose E. Marchesi  <jose.marchesi@oracle.com>

	PR gas/30690
	* config/tc-bpf.c (struct bpf_insn): Add fields is_relaxable and
	relaxed_exp.
	(enum options): Add OPTION_NO_RELAX.
	(md_longopts): Likewise for -mno-relax.
	(do_relax): New global.
	(md_parse_option): Handle OPTION_NO_RELAX.
	(RELAX_BRANCH_ENCODE): Define.
	(RELAX_BRANCH_P): Likewise.
	(RELAX_BRANCH_LENGTH): Likewise.
	(RELAX_BRANCH_CONST): Likewise.
	(RELAX_BRANCH_UNCOND): Likewise.
	(relaxed_branch_length): New function.
	(md_estimate_size_before_relax): Likewise.
	(read_insn_word): Likewise.
	(encode_int16): Likewise.
	(encode_int32): Likewise.
	(write_insn_bytes): Likewise.
	(md_convert_frag): Likewise.
	(encode_insn): Likewise.
	(install_insn_fixups): Likewise.
	(add_fixed_insn): Likewise.
	(add_relaxed_insn): Likewise.
	(md_assemble): Move instruction encoding logic to the above
	new functions.
	* testsuite/gas/bpf/jump-relax-ja.d: New test.
	* testsuite/gas/bpf/jump-relax-ja-be.d: Likewise.
	* testsuite/gas/bpf/jump-relax-ja.s: And corresponding source.
	* testsuite/gas/bpf/jump-relax-jump.d: New test.
	* testsuite/gas/bpf/jump-relax-jump-be.d: Likewise.
	* testsuite/gas/bpf/jump-relax-jump.s: And corresponding source.
	* testsuite/gas/bpf/bpf.exp: Run new tests.
	* doc/c-bpf.texi (BPF Options): Document -mno-relax.