perf tests: Fix data symbol test with LTO builds

perf tests: Fix data symbol test with LTO builds

With LTO builds, although regular builds could also see this as
all the code is in one file, the datasym workload can realize the
buf1.reserved data is never accessed. The compiler moves the
variable to bss and only keeps the data1 and data2 parts as
separate variables. This causes the symbol check to fail in the
test. Make the variable volatile to disable the more aggressive
optimization. Rename the variable to make which buf1 in perf is
being referred to.

Before:

  $ perf test -vv "data symbol"
  126: Test data symbol:
  --- start ---
  test child forked, pid 299808
  perf does not have symbol 'buf1'
  perf is missing symbols - skipping test
  ---- end(-2) ----
  126: Test data symbol                                                : Skip
  $ nm perf|grep buf1
  0000000000a5fa40 b buf1.0
  0000000000a5fa48 b buf1.1

After:

  $ nm perf|grep buf1
  0000000000a53a00 d buf1
  $ perf test -vv "data symbol"126: Test data symbol:
  --- start ---
  test child forked, pid 302166
   a53a00-a53a39 l buf1
  perf does have symbol 'buf1'
  Recording workload...
  Waiting for "perf record has started" message
  OK
  Cleaning up files...
  ---- end(0) ----
  126: Test data symbol                                                : Ok

Fixes: 3dfc01fe9d12 ("perf test: Add 'datasym' test workload")
Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250226230109.314580-1-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>