[libc] Document baremetal external API

[petrhosek]

LLVM libc baremetal implementation declares a set of external API that vendors are expected to provide, which libc uses to implement functionality that cannot be implemented in portable way. We should document this API (and perhaps also move all the declarations into a single header).

[llvmbot]

@llvm/issue-subscribers-libc

Author: Petr Hosek (petrhosek)

LLVM libc baremetal implementation declares a set of external API that vendors are expected to provide, which libc uses to implement functionality that cannot be implemented in portable way. We should document this API (and perhaps also move all the declarations into a single header).