btf: do not use the CHAR `encoding' bit for BTF

btf: do not use the CHAR `encoding' bit for BTF

Contrary to CTF and our previous expectations, as per [1], turns out
that in BTF:

1) The `encoding' field in integer types shall not be treated as a
   bitmap, but as an enumerated, i.e. these bits are exclusive to each
   other.

2) The CHAR bit in `encoding' shall _not_ be set when emitting types
   for char nor `unsigned char'.

Consequently this patch clears the CHAR bit before emitting the
variable part of BTF integral types.  It also updates the testsuite
accordingly, expanding it to check for BOOL bits.

[1] https://lore.kernel.org/bpf/a73586ad-f2dc-0401-1eba-2004357b7edf@fb.com/T/#t

gcc/ChangeLog:

	* btfout.cc (output_asm_btf_vlen_bytes): Do not use the CHAR
	encoding bit in BTF.

gcc/testsuite/ChangeLog:

	* gcc.dg/debug/btf/btf-int-1.c: Do not check for char bits in
	bti_encoding and check for bool bits.