[PATCH RFC] x86: remove toolchain check for X32 ABI capability

[Fangrui Song]

On 2021-02-28, Masahiro Yamada wrote:
>This commit reverts 0bf6276392e9 ("x32: Warn and disable rather than
>error if binutils too old").
>
>The help text in arch/x86/Kconfig says enabling the X32 ABI support
>needs binutils 2.22 or later. This is met because the minimal binutils
>version is 2.23 according to Documentation/process/changes.rst.
>
>I would not say I am not familiar with toolchain configuration, but
>I checked the configure.tgt code in binutils. The elf32_x86_64
>emulation mode seems to be included when it is configured for the
>x86_64-*-linux-* target.
>
>I also tried lld and llvm-objcopy, and succeeded in building x32 VDSO.
>
>I removed the compile-time check in arch/x86/Makefile, in the hope of
>elf32_x86_64 being always supported.
>
>With this, CONFIG_X86_X32 and CONFIG_X86_X32_ABI will be equivalent.
>Rename the former to the latter.

Hi Masahiro, the cleanup looks nice!

As of LLVM toolchain support, I don't know any user using LLVM binary
utilities or LLD.
The support on binary utitlies should be minimum anyway (EM_X86_64,
ELFCLASS32, ELFDATA2LSB are mostly all the tool needs to know for many utilities), so
many of they should just work.

For llvm-objcopy, I know two issues related to `$(OBJCOPY) -O elf32-x86-64`
(actually `objcopy -I elf64-x86-64 -O elf32-x86-64`).  Such an operation tries
to convert an ELFCLASS64 object file to an ELFCLASS32 object file. It is not very clear
what GNU objcopy does. llvm-objcopy is dumb and does not do fancy CLASS conversion.

* {gcc,clang} -gz{,=zlib} produced object files. The Elf{32,64}_Chdr headers are different.
   Seems that GNU objcopy can convert the headers (https://github.com/ClangBuiltLinux/linux/issues/514).
   llvm-objcopy cannot do it.
* Seems that GNU objcopy can convert .note.gnu.property (https://github.com/ClangBuiltLinux/linux/issues/1141#issuecomment-678798228)
   llvm-objcopy cannot do it.


On the linker side, I know TLS relaxations and IBT need special care and I
believe LLD does not handle them correctly. Thankfully the kernel does not use
thread-local storage so this is not an issue. So perhaps for most configurations
it is already working.  Since you've tested it, that is good news to me:)