Closes #1094 .

This used to be defined multiple times in different (but not all)
architectures. Moving it to general linux module since it's an
architecture independent feature.

These are used by the flock wrapper in rustc_data_structures. The
constants were already present in x86_64-linux-gnu and BSD (since
4928bd98).

`pthread_mutex_t` varies across architectures, in several ways:
- endianness alters the ordering of bytes, since the contents of the
  struct are larger than 8-bit.
- its length varies.
- the location of the mutex kind (`PTHREAD_MUTEX_RECURSIVE`,
  `PTHREAD_MUTEX_ERRORCHECK` or `PTHREAD_MUTEX_ADAPTIVE_NP`) varies
  between 32-bit and 64-bit: On 32-bit architectures, it is preceded by
  three int/unsigned int, while on 64-bit architectures, it is preceded
  by four of them.

These initializers are only available from <pthread.h> when _GNU_SOURCE
is defined.

Relax the cfg_if check in ci/style.rs to allow #[cfg(target_endian)]
tests.

Note that termios2 doesn't exist on powerpc(64), termios
is used instead.