perf pmu-events: Split big_c_string storage into standalone compilation unit

perf pmu-events: Split big_c_string storage into standalone compilation unit

Currently, jevents.py emits both the massive 2.8 MB big_c_string literal
and tens of thousands of compact_pmu_event struct arrays into a single
pmu-events.c compilation unit. Compiling this giant file takes ~2.2 seconds
on a single CPU core during Kbuild startup.

Refactor jevents.py to emit big_c_string into a dedicated
pmu-events-string.c compilation unit. This allows Kbuild to compile
pmu-events.o and pmu-events-string.o simultaneously in parallel across
two separate CPU cores, preserving 100% string deduplication and zero
dynamic ELF relocations while cutting C compilation latency in half.

Add pmu-events-string.c to tools/perf/.gitignore to ensure in-tree Kbuild
runs do not leave untracked generated files in the working directory.

To guarantee 100% backward compatibility with GNU Make 4.0+ (avoiding the
Make 4.3+ grouped target &: syntax which causes older Make versions like
4.2.1 to spawn multiple concurrent jevents.py processes during parallel
builds), implement a robust dependency chaining pattern:
  $(PMU_EVENTS_C): $(JEVENTS_DEPS)
  $(PMU_EVENTS_STRING_C): $(PMU_EVENTS_C)
          @:
This ensures jevents.py is invoked exactly once. If jevents.py aborts
early, Make's .DELETE_ON_ERROR: purges pmu-events.c, guaranteeing that
subsequent Make invocations correctly re-execute the script and overwrite
pmu-events-string.c. In jevents.py, explicitly close output_file first
and output_string_file second at the tail of main() to guarantee that
pmu-events-string.c receives a filesystem timestamp greater than or equal
to pmu-events.c, completely avoiding redundant incremental rebuilds.

Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Gemini:gemini-3.1-pro-preview
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: James Clark <james.clark@linaro.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Chartre <alexandre.chartre@oracle.com>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>
Cc: Collin Funk <collin.funk1@gmail.com>
Cc: Costa Shulyupin <costa.shul@redhat.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Dapeng Mi <dapeng1.mi@linux.intel.com>
Cc: David Sterba <dsterba@suse.com>
Cc: Dmitrii Dolgov <9erthalion6@gmail.com>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Howard Chu <howardchu95@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Leo Yan <leo.yan@arm.com>
Cc: Markus Mayer <mmayer@broadcom.com>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Terrell <terrelln@fb.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <pjw@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Monnet <qmo@kernel.org>
Cc: Ricky Ringler <ricky.ringler@proton.me>
Cc: Song Liu <song@kernel.org>
Cc: Swapnil Sapkal <swapnil.sapkal@amd.com>
Cc: Thomas Falcon <thomas.falcon@intel.com>
Cc: Tomas Glozar <tglozar@redhat.com>
Cc: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>