[BE] Allow disabling USE_MIMALLOC on Windows and AArch64 (#199064)

`set(USE_MIMALLOC ON)` created a normal variable that shadowed the cached option, so `-DUSE_MIMALLOC=OFF` was ignored on those platforms.
Compute the platform default into `USE_MIMALLOC_DEFAULT` and pass it to `option()` instead, following pattern established by `USE_LLVM_BOLT`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/199064
Approved by: https://github.com/Skylion007