Use -perm /a+x instead of -perm +a+x in calls to find

[Munksgaard]

The use of +a+x is deprecated.

Fixes #19981.

[alexcrichton]

This ended up failing on windows: http://buildbot.rust-lang.org/builders/auto-win-64-nopt-t/builds/1271/steps/test/logs/stdio

[Munksgaard]

Ah, well that ain't good. The current approach -perm +a+x fails for me though, on Gentoo. Maybe try something different?

$ find --version
find (GNU findutils) 4.5.14
Packaged by Gentoo (4.5.14-r1)
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Eric B. Decker, James Youngman, and Kevin Dalley.
Features enabled: D_TYPE O_NOFOLLOW(enabled) LEAF_OPTIMISATION FTS(FTS_CWDFD) CBO(level=2) 

Edit for clarification: find +a+x fails for me.