Manual for platform-specific features and new __ppc_get_timebase inline.

Manual for platform-specific features and new __ppc_get_timebase inline.

[BZ #13743]
A new class of installed headers has been documented for low-level
platform-specific functionality.  PowerPC added the first instance with a
function to provide time base register access (__ppc_get_timebase).  This
is required for applications that measure time at high frequencies with
high precision that can't afford a syscall.