fix new/delete overrides

[Noxybot]

apparently, even if MI_USE_CXX is OFF, mimalloc still provides mangled definitions of new/delete operators. It causes inability to statically link mimaloc in our scenario, since we want to have our own overrides of new/delete functions.

[res2k]

Uh, but wouldn't this change disable C++ new/delete overrides entirely when not compiling as C++?

The way MI_USE_CXX is described, and the fact that there are explicitly mangled symbols, suggests that "compiles as pure C, but provides C++ overrides" is a quite deliberately chosen scenario, and that those overrides are not supposed to be available only when MI_USE_CXX is enabled.
For one, I believe that, usually, C malloc/free and C++ new/delete are backed by the same allocator; so doing the same for overrides is probably the safe options.
And secondly, but more importantly, simply disabling the code when MI_USE_CXX is off will break any users that rely on the "built with C, overrides C++" behavior.

In other words: disabling the C++ overrides probably warrants a separate build option/preprocessor macro to check.

[Noxybot]

Uh, but wouldn't this change disable C++ new/delete overrides entirely when not compiling as C++?

Yes. But it was my anticipation from an option called MI_USE_CXX being set to OFF.

Why would we ever want to override global new/delete in C only code? It's C-only code, so those new/delete are not from C standard library.