[llvm-objcopy][NFC] Move ownership keeping code into restoreStatOnFile().

The D93881 added functionality which preserve ownership for output file
if llvm-objcopy is called under root. That code was added into the place
where output file is created. The llvm-objcopy already has a function which
sets/restores rights/permissions for the output file.
That is the restoreStatOnFile() function. This patch moves code
(preserving ownershipping) into the restoreStatOnFile() function.

Differential Revision: https://reviews.llvm.org/D98511