Add support for making functions const

[Aaron1011]

PR rust-lang/rust#64906 adds the ability to write const extern fn and const unsafe extern fn, which will allow manys functions in libc to become const.

This is particuarly useful for functions which correspond to C macros (e.g. CMSG_SPACE). In C, these macros are constant expressions, allowing them to be used when declaring arrays. However, since the corresponding libc functions are not const, writing equivalent Rust code is impossible. Users must either perform an unecessary heap allocation, or pull in bindgen to evaluate the macro for specific values (e.g. CMSG_SPACE(1)).

However, the syntax const extern fn is not currently parsed by rust. To allow libc to use this without breaking backwards compatibility (i.e. bumping the minimum Rust version), I've taken the following approach:

1. A new off-by-default feature extern-const-fn is added to libc.
2. The internal f! macro has two versions, selected at compile-time by a cfg_if. When extern-const-fn is enabled, the declared f! macro passes through the const keyword from the macro user to the final definition (pub const unsafe extern fn foo. When extern-const-fn is disabled, the const keyword passed by the macro user is discarded, resulting in a plain pub extern const fn being declared.

Unfortunately, I couldn't manage to get macro_rules to accept a normal const token in the proper place (after pub). I had to resort to placing it in curly brackets:

pub {const} fn foo(val: u8) -> i8 {
}

The f! macro then translates this to a function definition with const in the proper position.

I'd appreciate it if someone who's more familiar with macro_rules! could see if I missed a way to get the desired syntax.