powerpc64: maintain own linux/powerpc64 subdirectory

powerpc64: maintain own linux/powerpc64 subdirectory

Most architectures supported by strace have their own subdirectory in
linux directory, but powerpc64 didn't, causing inconveniences.

* configure.ac (arch): Define to "powerpc64" on powerpc64.
* linux/powerpc/errnoent1.h: Move to linux/powerpc64/.
* linux/powerpc/ioctlent1.h: Likewise.
* linux/powerpc/signalent1.h: Likewise.
* linux/powerpc/syscallent1.h: Likewise.
* linux/powerpc64/ioctlent.h: New file.
* linux/powerpc64/syscallent.h: Likewise.
* linux/powerpc64/userent.h: Likewise.
* Makefile.am (EXTRA_DIST): Remove linux/powerpc/ioctlent1.h,
linux/powerpc/syscallent1.h, linux/powerpc/errnoent1.h, and
linux/powerpc/signalent1.h; added linux/powerpc64/errnoent1.h,
linux/powerpc64/ioctlent.h, linux/powerpc64/ioctlent1.h,
linux/powerpc64/signalent1.h, linux/powerpc64/syscallent.h,
linux/powerpc64/syscallent1.h, and linux/powerpc64/userent.h.