[llvm] MCFragment: Use trailing data for fixed-size part (PR #150846)

[Fangrui Song via llvm-commits]

MaskRay wrote:

While we could replace `uint32_t VarFixupEnd` with `uint8_t VarFixupSize` to make `MCFragment` smaller, I will try to make fixups detached from MCFragment.

https://github.com/llvm/llvm-project/pull/150846