The wcstombs() function conforms to ISO/IEC 9899:1999 (``ISO C99'')

In preparation for rust-lang/rust#37545 this is adding the appropriate
directives to libc to get included.

strnlen is used to find the length of a C string that may be
lacking a terminal NUL character. Whilst it is possible to
implement the equivalent functionality in rust code, it is
cleaner, simpler and removes the need for duplication of tricky
functionality to get right. It also makes it easier to port
C code snippets to rust.

Note that strnlen is not part of POSIX or C99.

This commit changes most uses of cfg_if as follows:
- fallthrough `else` usage is avoided for architecture or OS specific
  items
- a comment is added in the final `else` clause to signal intent someone
  modifying

It is safer to omit items than include ones for the wrong platform or
architecture.

The Rumprun linker wrapper already includes -lc when linking. Passing it
twice unfortunately causes binutils to crash with an assertion failure.

Rumprun does currently not provide librt.

This adds a `use_std` Cargo feature which disables `#![no_std]` builds of libc,
but is enabled by default. The library will currently continue to link to the
standard library to maintain backwards compatibility with the 0.2 series and
older Rust compilers for now, but this default can possible be changed in the
future.

This is defined by the C standard to indicate abnormal program
terminaiton; it is defined to call raise(SIGABRT), and to not return.
This can be useful when panics are not an option, such as signal
handlers handling stack overflow.

Also document them to prevent future regressions

Not gonna gain much from issuing warnings about various aspects! This is managed
separately from the standard library

They're all added under the `foo64` names with the `foo64` types added as well,
so they still need to be explicitly chosen.