bootstrap.bash: preserve file times when copying

Preserve file modification times when copying bootstrap tree,
this makes generated bootstrap more friendly to rsyncing.

Change-Id: I32cde58c25b48d3c00d4413860dbd49a265b0ff2
Reviewed-on: https://go-review.googlesource.com/c/go/+/174217Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bootstrap.bash

@@ -45,7 +45,7 @@ fi
 unset GOROOT
 src=$(cd .. && pwd)
 echo "#### Copying to $targ"
-cp -R "$src" "$targ"
+cp -Rp "$src" "$targ"
 cd "$targ"
 echo
 echo "#### Cleaning $targ"