[lld] [llvm] Rename `F_no_mmap` to `F_mmap` (PR #134787)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

In future please include the *why* in the commit message, not just the *what*. There is no explanation for why this way round is better.

https://github.com/llvm/llvm-project/pull/134787