(no_)global_oom_handling should be a feature

[remexre]

It'd be helpful if I could get allocator-api2 to behave as if cfg(no_global_oom_handling) was set, without actually having to switch to nightly, rebuild libcore/liballoc, etc. for it.

I think this might best be done by:

• Adding an enabled-by-default global_oom_handling feature
• Switching existing #[cfg(not(no_global_oom_handling))]s to #[cfg(all(not(no_global_oom_handling), feature = "global_oom_handling"))]
• Maybe adding a pub const somewhere to expose whether the feature ended up being enabled (I want to assert elsewhere that it was not).