std: reorganize the UNIX-internal `weak` module
This moves the `dlsym`-based and weak-linkage versions of the `weak!` macro into separate files, both of which include a common test file. As a result, both versions will be tested on all the platforms where they are used.
Since the `#[link_name]` arm of the `dlsym` version was unused, I've removed it. I've also removed the unused `raw_syscall!` and non-Linux `syscall!` macros and gated the `#[allow(dead_code, unused_macros)]` to only apply on non-Linux platforms, so compilation will fail if `weak` turns out to be unused on all platforms.
The last change concerns the use of `dlsym!` on FreeBSD: it is only used once, to link against `sysctlbyname`. But that symbol is always available, so there is no need for weak linkage.
e93ec9aab4