Introduce constant function for further compiler optimizations

[jserv]

GNU extension __attribute__ ((const)) helps the compiler to know semantic
meaning of a function call, so that it allows higher optimization than
to normal functions. For example, Common Sub-expression Elimination
(CSE) is used to avoid duplicating the same code inside a function, and
function _mi_os_page_size can benefit from CSE optimization once it is
declared as a constant function.

Reference:
https://lwn.net/Articles/285332/

[daanx]

Thanks @jserv ! Right, I had been thinking about const and pure attributes. However, it is also tricky as we really need to make sure the annotation is correct -- if not, things can subtly go very wrong at runtime.

I think most annotations in your PR are correct, but actually the one for os_page_size is not I think -- the function has no arguments and it reads from "global" memory; indeed, the global variable is initialized at some point so it could return different values at different points (of course, we carefully (try to) ensure that we only read it after initializiation but the compiler does not know that. I guess it may work out but I think it is still incorrect -- the pure annotation is correct I guess ?

(also, we should add void to the os_page_size declaration.)

(so, overall I am hesitant with these annotations and we need to check with lots of care)