BTF: Support for BTF_KIND_FLOAT

BTF: Support for BTF_KIND_FLOAT

Add BTF_KIND_FLOAT, a new BTF type kind which has recently stabilized in
the linux kernel [1]. This kind is used for encoding floating point
types, and is of particular use when generating BTF for some s390
arch-specific kernel headers.

Also update some BTF tests which previously used floating point types to
check correct behavior for types with no BTF representation.

[1]: https://github.com/torvalds/linux/commit/b1828f0b04828aa8cccadf00a702f459caefeed9

include/ChangeLog:

	* btf.h (struct btf_type): Update bit usage comment.
	(BTF_INFO_KIND): Update bit mask.
	(BTF_KIND_FLOAT): New define.
	(BTF_KIND_MAX): Update.

gcc/ChangeLog:

	* btfout.c (get_btf_kind): Support BTF_KIND_FLOAT.
	(btf_asm_type): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.dg/debug/btf/btf-float-1.c: New test.
	* gcc.dg/debug/btf/btf-function-3.c: Use different unrepresentable type.
	* gcc.dg/debug/btf/btf-struct-2.c: Likewise.
	* gcc.dg/debug/btf/btf-variables-2.c: Likewise.