[Bug target/110096] Would be nice if __builtin_ia32_pause had a portable equivalent as it's applicable to ARM

[pdimov at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110096

--- Comment #7 from Peter Dimov <pdimov at gmail dot com> ---
These intrinsics are typically used in spinlocks as in
```
while( flag_.test_and_set() )
{
    // issue a power-saving NOP here
}
```
(where `flag_` is `std::atomic_flag`) and this use is generic and not
target-dependent.