Flush executables to disk after linkage

[liranringel]

I read that: https://randomascii.wordpress.com/2018/02/25/compiler-bug-linker-bug-windows-kernel-bug/amp/
The author suggests to ensure that the binary is flushed to disk after linkage.
We could use File::sync_all which eventually calls FlushFileBuffers here:

rust/src/librustc_trans/back/link.rs

Line 822 in 322d7f7

fn exec_linker(sess: &Session, cmd: &mut Command, tmpdir: &Path)