perf test: Add 'sqrtloop' test workload

The sqrtloop creates a child process to run an infinite loop calling
sqrt() with rand().  This is needed for ARM SPE fork test.

  $ perf test -w sqrtloop

It can take an optional argument to specify how long it will run in
seconds (default: 1).

Committer notes:

Explicitely ignored the sqrt() return to fix the build on systems where
the compiler complains it isn't being used.

And added a sqrtloop specific CFLAGS to disable optimizations to make
this a bit more robust wrt dead code elimination.

Doing that a -U_FORTIFY_SOURCE needs to be added, as -O0 is incompatible
with it.

Signed-off-by: Namhyung Kim <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Athira Jajeev <[email protected]>
Cc: German Gomez <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Zhengjun Xing <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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