[Bug target/93492] Broken code with -fpatchable-function-entry and -fcf-protection=full

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93492

--- Comment #22 from Iain Sandoe <iains at gcc dot gnu.org> ---
I looked yesterday and, as of Xcode 11.5, the patchable_function_entry
attribute is not recognised but the system tools.

That means, for now, that there is no de-facto ABI for patchable functions on
Mach-O/Darwin platforms (regardless of the fact that upstream clang accepts the
attribute for Darwin targets).

The "old" patchable function option (-mfix-and-continue) is recognised but is a
NOP for modern Darwin.

My plan is to skip the test(s) on Darwin - if there are enough test(s) then we
should have a target-supports for the facility.

I note in passing thet the target hook is not terribly friendly - the syntax
error on Darwin is only because of the hard-wired assumption that named
sections are ELF-named-sections.  Which means that we will (when the ABI is
settled) have to re-write the whole hook potentially for one line of code
difference.

In general, for a compiler like GCC that is supposed to support many targets,
it would be a Good Idea is folks would not assume that everything is ELF.