perf parse-events: Add "cpu" term to set the CPU an event is recorded on

perf parse-events: Add "cpu" term to set the CPU an event is recorded on

The -C option allows the CPUs for a list of events to be specified but
its not possible to set the CPU for a single event. Add a term to
allow this. The term isn't a general CPU list due to ',' already being
a special character in event parsing instead multiple cpu= terms may
be provided and they will be merged/unioned together.

An example of mixing different types of events counted on different CPUs:
```
$ perf stat -A -C 0,4-5,8 -e "instructions/cpu=0/,l1d-misses/cpu=4,cpu=5/,inst_retired.any/cpu=8/,cycles" -a sleep 0.1

 Performance counter stats for 'system wide':

CPU0            6,979,225      instructions/cpu=0/              #    0.89  insn per cycle
CPU4               75,138      cpu/l1d-misses/
CPU5            1,418,939      cpu/l1d-misses/
CPU8              797,553      cpu/inst_retired.any,cpu=8/
CPU0            7,845,302      cycles
CPU4            6,546,859      cycles
CPU5          185,915,438      cycles
CPU8            2,065,668      cycles

       0.112449242 seconds time elapsed
```

Committer testing:

  root@number:~# grep -m1 "model name" /proc/cpuinfo
  model name	: AMD Ryzen 9 9950X3D 16-Core Processor
  root@number:~# perf stat -A -e "instructions/cpu=0/,instructions,l1d-misses/cpu=4,cpu=5/,cycles" -a sleep 0.1

   Performance counter stats for 'system wide':

  CPU0    2,398,351   instructions/cpu=0/    #  0.44  insn per cycle
  CPU0    2,398,152   instructions           #  0.44  insn per cycle
  CPU1    1,265,634   instructions           #  0.49  insn per cycle
  CPU2      606,087   instructions           #  0.50  insn per cycle
  CPU3    4,025,752   instructions           #  0.52  insn per cycle
  CPU4    4,236,810   instructions           #  0.53  insn per cycle
  CPU5    3,984,832   instructions           #  0.66  insn per cycle
  CPU6      434,132   instructions           #  0.44  insn per cycle
  CPU7       65,752   instructions           #  0.41  insn per cycle
  CPU8      459,083   instructions           #  0.48  insn per cycle
  CPU9    6,464,161   instructions           #  1.31  insn per cycle
  <SNIP>
  root@number:~# perf stat -e "instructions/cpu=0/,instructions,l1d-misses/cpu=4,cpu=5/,cycles" -a sleep 0.

   Performance counter stats for 'system wide':

             144,822      instructions/cpu=0/              #    0.03  insn per cycle
           4,666,114      instructions                     #    0.93  insn per cycle
               2,583      l1d-misses
           4,993,633      cycles

         0.000868512 seconds time elapsed

  root@number:~#

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Kan Liang <kan.liang@linux.intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dominique Martinet <asmadeus@codewreck.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Weilin Wang <weilin.wang@intel.com>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Link: https://lore.kernel.org/r/20250403194337.40202-5-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>