The target triple name was changed in rust-lang/rust#106429

While `rustc` known the old spelling of the triple, `rustup` does not,
so the toolchain download was not working.

Signed-off-by: Joe Richey <joerichey@google.com>

This supersedes #341, and makes the following changes
  - All the code for implementing `__getrandom_custom` is now in an
    **named** `const` block (unnamed consts require Rust 1.37)
    - I found this approch [here](https://internals.rust-lang.org/t/anonymous-modules/15441)
    - Nothing inside the block can be referenced outside of it
  - `__getrandom_custom` is marked `unsafe`
    - It can't be accessed externally, but is "logically" unsafe as it
      dereferences raw pointers
  - The type of the function is moved to a typedef, so we can check that
    the defined type matches that of `getrandom:getrandom`.
  - Use `::core::result::Result` instead of `Result`
    - Similar to use use of `from_raw_parts_mut` this prevents
      compilation errors if `Result` is redefined.

Signed-off-by: Joe Richey <joerichey@google.com>