nptl: Add struct_rwlock.h

nptl: Add struct_rwlock.h

This patch adds a new generic __pthread_rwlock_arch_t definition meant
to be used by new ports.  Its layout mimics the current usage on some
64 bits ports and it allows some ports to use the generic definition.
The arch __pthread_rwlock_arch_t definition is moved from
pthreadtypes-arch.h to another arch-specific header (struct_rwlock.h).

Also the static intialization macro for pthread_rwlock_t is set to use
an arch defined on (__PTHREAD_RWLOCK_INITIALIZER) which simplifies its
implementation.

The default pthread_rwlock_t layout differs from current ports with:

  1. Internal layout is the same for 32 bits and 64 bits.

  2. Internal flag is an unsigned short so it should not required
     additional padding to align for word boundary (if it is the case
     for the ABI).

Checked with a build on affected abis.

Change-Id: I776a6a986c23199929d28a3dcd30272db21cd1d0