cmd/link: call syscall.FlushFileBuffers on outbuf Unmap
In the windows version of OutBuf.munmap, call syscall.FlushFileBuffers
after the call to syscall.FlushViewOfFile, on the theory that this
will help flush all associated meta-data for the file the linker is
writing.
Updates #44817.
Change-Id: Ibff7d05008a91eeed7634d2760153851e15e1c18
Reviewed-on: https://go-review.googlesource.com/c/go/+/406814 Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Alex Brainman <alex.brainman@gmail.com>