gas: rework timestamp preservation on doc/asconfig.texi

gas: rework timestamp preservation on doc/asconfig.texi

PR 28909

Sadly "cp -p", doing more than just preserving the time stamp, can fail
e.g. upon trying to preserve ownership (which we don't care about), as
can be observed on e.g. Cygwin. Replace the use of -p by a use of touch,
this way also only preserving modification time.