`dl_iterate_phdr()` and ELF friends only defined on Linux and Fuchsia.

Hi,

I'm writing a library around the dl_iterate_phdr API, and want it to be portable.

I've noticed that dl_iterate_phdr, its types, and constants, are defined only on Linux and Fuchsia for now:

$ ag PT_NULL
src/fuchsia/mod.rs
2431:pub const PT_NULL: u32 = 0;

src/unix/notbsd/linux/mod.rs
1225:pub const PT_NULL: u32 = 0;

$ ag dl_iterate_phdr
src/fuchsia/mod.rs
3976:    pub fn dl_iterate_phdr(

src/unix/notbsd/linux/mod.rs
2045:    pub fn dl_iterate_phdr(   

$ ag Elf64_Half                       
src/fuchsia/mod.rs
50:pub type Elf64_Half = u16;
930:        pub dlpi_phnum: Elf64_Half,

src/unix/notbsd/linux/mod.rs
31:pub type Elf64_Half = u16;
472:        pub dlpi_phnum: Elf64_Half,

AFAIK, all of this stuff if present on the BSDs. The ELF stuff is a standard ABI (although different for 32/64-bit arches) that could be shared across all platforms, but some of the types for dl_iterate_phdr differ per-platform. For example, on OpenBSD dl_phdr_info contains more fields than on Linux.

For now users can work around this by using bindgen, but in the long run I think it would be best for libc to implement these interfaces across all platforms.

Thanks

Start by comparing the existing ELF and dl_iterate_phdr definitions in src/fuchsia/mod.rs and src/unix/notbsd/linux/mod.rs with the corresponding BSD platform APIs. Trace the platform modules and account for differences such as OpenBSD's additional dl_phdr_info fields; done means the interfaces are exposed across the applicable BSD targets with platform-correct definitions.