perf/arch: Record sample last_period before updating on the x86 and PowerPC platforms

This change alters the PowerPC and x86 driver implementations to record
the last sample period before the event is updated for the next period.

A common pattern in PMU driver implementations is to have a
"*_event_set_period" function which takes care of updating the various
period-related fields in a perf_event structure. In most cases, the
drivers choose to call this function after initializing a sample data
structure with perf_sample_data_init. The x86 and PowerPC drivers
deviate from this, choosing to update the period before initializing the
sample data. When using an event with an alternate sample period, this
causes an incorrect period to be written to the sample data that gets
reported to userspace.

Signed-off-by: Mark Barnett <mark.barnett@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250408171530.140858-2-mark.barnett@arm.com

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