[X86] Fix i128 argument passing under SysV ABI

[tgross35]

Bring https://reviews.llvm.org/D158169 into Rust's tree, cherry picked from fa1b6e6 which will be present in LLVM18.

Author: Nikita Popov <npopov@redhat.com>
Date:   Thu Aug 17 11:55:38 2023 +0200

    [X86] Fix i128 argument passing under SysV ABI

    The x86_64 SysV ABI specifies that __int128 is passed either in
    two registers (if available) or in a 16 byte aligned stack slot.
    GCC implements this behavior. However, if only one free register
    is available, LLVM will instead pass one half of the i128 in a
    register, and the other on the stack.

    Make sure that either both are passed in registers or both on the
    stack.

    Fixes https://github.com/llvm/llvm-project/issues/41784.
    The patch is basically what craig.topper proposed to do there.

    Differential Revision: https://reviews.llvm.org/D158169

Combined with rust-lang/rust#116672, this will resolve i128 ABI problems.

MCP with more details: rust-lang/compiler-team#683

[tgross35]

r? @nikic

[nikic]

We cannot backport this ABI-changing patch. This part of the fix will have to wait for LLVM 18.