hwf: add detection of PowerPC hardware features

* src/Makefile.am: PowerPC hardware detection.
* src/g10lib.h: Likewise.
* src/hwf-common.h: Likewise.
* src/hwf-ppc.c: Likewise.
* src/hwfeatures.c: Likewise.
* configure.ac: Likewise.

[jk: split PowerPC HW features to separate patch, from
https://lists.gnupg.org/pipermail/gcrypt-devel/2019-July/004769.html]
[jk: disable __builtin_cpu_supports usage for now]

• GnuPG-bug-id: T4529
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>