gas: bpf: do not allow referring to register names as symbols in operands

2023-11-18  Jose E. Marchesi  <jemarch@gnu.org>

	* config/tc-bpf.c (parse_bpf_register): Move before
	bpf_parse_name.
	(bpf_parse_name): Do not allow using symbols that are also
	register names as operands in pseudo-c syntax.
	* testsuite/gas/bpf/regs-for-symbols-pseudoc.d: New file.
	* testsuite/gas/bpf/regs-for-symbols-pseudoc.s: Likewise.
	* testsuite/gas/bpf/regs-for-symbols-pseudoc.l: Likewise.
	* doc/c-bpf.texi (BPF Registers): Document that it is not possible
	to refer to register names as symbols in instruction operands.