[compiler-rt] Avoid partial munmap (PR #92109)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The new argument to `UnmapOrDie` will match how `ReadFileToBuffer` calls `UnmapOrDie`, so LGTM. But I think it is worth an emscripten bug report.

https://github.com/llvm/llvm-project/pull/92109