gh-127604: Optimize -ldl usage on platforms that use dlopen for libFFI.

[freakboy3742]

#133040 modified the handling of -ldl, resulting in multiple copies of -ldl being included in link commands.

#133071 modified this handling to minimise a lot of those usages; but on platforms that use dlopen() (macOS and iOS), there was still some duplicated use. This PR cleans up that usage.

• Issue: Add a way of printing a C backtrace to faulthandler #127604

[freakboy3742]

!buildbot iOS

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @freakboy3742 for commit 297f242 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F133081%2Fmerge

The command will test the builders whose names match following regular expression: iOS

The builders matched are:

• iOS ARM64 Simulator PR

[freakboy3742]

Nope... looks like that hasn't done it. I'll take another look in the morning if nobody beats me to it.

[picnixz]

It looks fine and I'm pretty sure there is a cleaner and alternative way to do it but I'm also pretty sure that it would require more steps and a more complicate configuration so I think we should live with this first.

More generally, I think we need to have a function that cleans up the flags to remove duplicate ones.

[picnixz]

Nope... looks like that hasn't done it. I'll take another look in the morning if nobody beats me to it.

Wait how come? then where is the one coming from...?

Apparently it doesn't work.

[picnixz]

Actually, it looks like it worked because before we had (in the "Compile build Python" step):

ld: warning: ignoring duplicate libraries: '-ldl'
ld: warning: ignoring duplicate libraries: '-ldl'
ld: warning: ignoring duplicate libraries: '-ldl'
ld: warning: ignoring duplicate libraries: '-ldl'
ld: warning: ignoring duplicate libraries: '-ldl'
ld: warning: ignoring duplicate libraries: '-ldl'

and now we have

ld: warning: ignoring duplicate libraries: '-ldl'
ld: warning: ignoring duplicate libraries: '-ldl'
ld: warning: ignoring duplicate libraries: '-ldl'
ld: warning: ignoring duplicate libraries: '-ldl'

So two warnings go eliminated. It remains to check what the others are. We also have 2 warnings less in the "Compile host Python" step.

[picnixz]

Note that the duplicate -ldl appear in only one command:

gcc -ldl    -o _bootstrap_python Modules/getbuildinfo.o [...] \
		Programs/_bootstrap_python.o Modules/getpath.o -ldl  -framework CoreFoundation