Split out architecture-dependent definitions into api.h and arch.h.

Break out architecture-specific definitions into api.h and arch.h.
Modify Makefile to add pthreads-x86 and pthreads-ppc targets to adapt
to either architecture.  Create api_x86.h, api_ppc.h, arch_x86.h, and
arch_ppc.h files.

The api/arch distinction is historical in nature.  In a perfect world,
these would be merged.  In reality, the __thread storage class does not
adapt nicely to accessing one thread's variables from another thread,
though sufficiently insane gcc hackery could probably make this work.
In the event, arch.h manually constructs an array of pointers to the
__thread variables that need cross-thread access, while api.h punts
and uses an array for the per-thread variables themselves, paying a
significant performance penalty for so doing.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>