gas: support for the BPF pseudo-c assembly syntax

gas: support for the BPF pseudo-c assembly syntax

This patch adds support to the GNU assembler for an alternative
assembly syntax used in BPF.  This syntax is C-like and very
unconventional for an assembly language, but it is generated by
clang/llvm and is also used in inline asm templates in kernel code, so
we ought to support it.

After this patch, the assembler is able to parse instructions in both
supported syntax: the normal assembly-like syntax and the pseudo-C
syntax.  Instruction formats can be mixed in the source program: the
assembler recognizes the right syntax to use.

gas/ChangeLog:

2023-04-20  Guillermo E. Martinez  <guillermo.e.martinez@oracle.com>

	PR gas/29728
	* config/tc-bpf.h (TC_EQUAL_IN_INSN): Define.
	* config/tc-bpf.c (LEX_IS_SYMBOL_COMPONENT): Define.
	(LEX_IS_WHITESPACE): Likewise.
	(LEX_IS_NEWLINE): Likewise.
	(LEX_IS_ARITHM_OP): Likewise.
	(LEX_IS_STAR): Likewise.
	(LEX_IS_CLSE_BR): Likewise.
	(LEX_IS_OPEN_BR): Likewise.
	(LEX_IS_EQUAL): Likewise.
	(LEX_IS_EXCLA): Likewise.
	(ST_EOI): Likewise.
	(MAX_TOKEN_SZ): Likewise.
	(init_pseudoc_lex): New function.
	(md_begin): Call init_pseudoc_lex.
	(valid_expr): New function.
	(build_bpf_non_generic_load): Likewise.
	(build_bpf_atomic_insn): Likewise.
	(build_bpf_jmp_insn): Likewise.
	(build_bpf_arithm_insn): Likewise.
	(build_bpf_endianness): Likewise.
	(build_bpf_load_store_insn): Likewise.
	(look_for_reserved_word): Likewise.
	(is_register): Likewise.
	(is_cast): Likewise.
	(get_token): Likewise.
	(bpf_pseudoc_to_normal_syntax): Likewise.
	(md_assemble): Try pseudo-C syntax if an instruction cannot be
	parsed.