Add support for building s390 with `LLVM=1`

[nathanchance]

LLD has gained s390 support in main (and hopefully 18.1.0):

llvm/llvm-project@fe3406e
llvm/llvm-project#81675

There were several issues uncovered on the Linux side that needed to be fixed:

ClangBuiltLinux/linux#1991
ClangBuiltLinux/linux#1992
ClangBuiltLinux/linux#1993
ClangBuiltLinux/linux#1994
ClangBuiltLinux/linux#1995

Once all of those changes are available in -next and apt.llvm.org has been updated to include that LLVM change, we should cut all of the relevant LLVM builds on -next over to linking with ld.lld. Ideally, we would just cut over to LLVM=1 OBJCOPY=s390x-linux-gnu-objcopy OBJDUMP=s390x-linux-gnu-objdump, as that is known to work with all of the configurations that we test.

[nathanchance]

I have updated the title to reflect that:

• llvm-objcopy works with the latest LLVM release/18.x (llvm/llvm-project@5ef297a) and main (llvm/llvm-project@3c02cb7) branches and the current s390 features branch (https://git.kernel.org/s390/c/9ea30fd166e9b869546059f714beab78cb150b11)
• llvm-objdump will work with the s390 features branch once https://lore.kernel.org/linux-s390/20240220-s390-work-around-llvm-objdump-t-j-v1-1-47bb0366a831@kernel.org/ has been pushed out (https://lore.kernel.org/linux-s390/20240221091612.7561-A-hca@linux.ibm.com/)

With this, we can cut over to full LLVM=1 with ARCH=s390 with Linux trees and LLVM toolchains that satisfy these conditions (we will have to validate the versions of LLVM that tuxsuite has available).

[nathanchance]

#712