bpf: gas: consolidate handling of immediate overflows

bpf: gas: consolidate handling of immediate overflows

This commit changes the BPF GAS port in order to handle immediate
overflows the same way than the clang BPF assembler:

- For an immediate field of N bits, any written number (positive or
  negative) whose two's complement encoding fit in N its is accepted.
  This means that -2 is the same than 0xffffffe.  It is up to the
  instructions to decide how to interpret the encoded value.

- Immediate fields in jump instructions are no longer relaxed.
  Relaxing to jump instructions with wider range is only performed
  when expressions are involved.

- The manual is updated to document this, and testsuite adapted
  accordingly.

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

gas/ChangeLog:

2023-08-17  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* config/tc-bpf.c (check_immediate_overflow): New function.
	(encode_insn): Use check_immediate_overflow.
	(md_assemble): Do not relax instructions with
	constant disp16 fields.
	* doc/c-bpf.texi (BPF Instructions): Add note about how numerical
	literal values are interpreted for instruction immediate operands.
	* testsuite/gas/bpf/disp16-overflow.s: Adapt accordingly.
	* testsuite/gas/bpf/jump-relax-jump.s: Likewise.
	* testsuite/gas/bpf/jump-relax-jump.d: Likewise.
	* testsuite/gas/bpf/jump-relax-jump-be.d: Likewise.
	* testsuite/gas/bpf/jump-relax-ja.s: Likewise.
	* testsuite/gas/bpf/jump-relax-ja.d: Likewise.
	* testsuite/gas/bpf/jump-relax-ja-be.d: Likewise.
	* testsuite/gas/bpf/disp16-overflow-relax.l: Likewise.
	* testsuite/gas/bpf/imm32-overflow.s: Likewise.
	* testsuite/gas/bpf/disp32-overflow.s: Likewise.
	* testsuite/gas/bpf/disp16-overflow.l: Likewise.
	* testsuite/gas/bpf/disp32-overflow.l: Likewise.
	* testsuite/gas/bpf/imm32-overflow.l: Likewise.
	* testsuite/gas/bpf/offset16-overflow.l: Likewise.