gas: elf: binutils: add new section type SHT_GNU_SFRAME

gas: elf: binutils: add new section type SHT_GNU_SFRAME

So far, SFrame sections were of type SHT_PROGBITS.

As per ELF specification, SHT_PROGBITS indicates that the section holds
information defined by the program, whose format and meaning are
determined solely by the program.

On the linker side, SHT_PROGBITS should be reserved for the simple "cat
contents after applying relocs" semantics.

Currently, the only way to know that a section contains SFrame stack
trace data is if consumer checks for section name.  Such a check for
section name is not quite conformant to ELF principles.

Some of this was discussed here
https://sourceware.org/pipermail/binutils/2025-March/140181.html

With this change, the SFrame sections generated by gas, ld will have
section type set to SHT_GNU_SFRAME.   The new section type is defined in
the SHT_LOOS/SHT_HIOS space.  The SFrame parsing routine
_bfd_elf_parse_sframe () now admits sections only when the the section
type is SHT_GNU_SFRAME.

No special handling / validation is done at the moment for the case of
manual creation of SFrame sections via obj_elf_section ().  Add function
level comments for now to add a note about this.

Although the default handling for (sh_type >= SHT_LOOS && sh_type <=
SHT_HIOS) is sufficient when SHT_GNU_SFRAME is in that range, it makes
sense to add it as a case of its own.

bfd/
	* elf-sframe.c (_bfd_elf_parse_sframe): Check if section type is
	SHT_GNU_SFRAME.
	(_bfd_elf_set_section_sframe): Set SHT_GNU_SFRAME for output
	SFrame section.
	* elflink.c (obj_elf_section): Use section type for check
	instead of section name.
	* elfxx-x86.c: Set SHT_GNU_SFRAME for SFrame sections for
	.plt* sections.
	* elf.c (bfd_section_from_shdr): Add case for SHT_GNU_SFRAME.
binutils/
	* readelf.c (get_os_specific_section_type_name): Add
	SHT_GNU_SFRAME.
gas/
	* NEWS: Announce emitted SFrame sections have SHT_GNU_SFRAME
	set.
	* config/obj-elf.c (obj_elf_attach_to_group): Add comments to
	indicate no special handling for SFrame yet.
	* dw2gencfi.c (cfi_finish): Set SHT_GNU_SFRAME for emitted
	SFrame section.
ld/
	* NEWS: Announce emitted SFrame sections have SHT_GNU_SFRAME
	set.
gas/testsuite/
	* gas/cfi-sframe/cfi-sframe.exp: Add new test.
	* gas/cfi-sframe/cfi-sframe-common-1b.d: New test.
	* gas/cfi-sframe/cfi-sframe-common-1b.s: New test.
include/
	* elf/common.h (SHT_GNU_SFRAME): Add new section type for SFrame
	stack trace information.
libsframe/doc/
	* sframe-spec.texi: Add expected ELF section type.