Auto merge of #116672 - maurer:128-align, r=nikic

LLVM 18 x86 data layout update

With https://reviews.llvm.org/D86310 LLVM now has i128 aligned to 16-bytes on x86 based platforms. This will be in LLVM-18. This patch updates all our spec targets to be 16-byte aligned, and removes the alignment when speaking to older LLVM.

This results in Rust overaligning things relative to LLVM on older LLVMs.

This implements MCP rust-lang/compiler-team#683.

See #54341

Author: bors