perf tools: Use Python devtools for version autodetection rather than runtime

This fixes the issue where the build will fail if only the Python2
runtime is installed but the Python3 devtools are installed. Currently
the workaround is 'make PYTHON=python3'.

Fix it by autodetecting Python based on whether python[x]-config exists
rather than just python[x] because both are needed for the build. Then
-config is stripped to find the Python runtime.

Testing
=======

 * Auto detect links with Python3 when the v3 devtools are installed
   and only Python 2 runtime is installed
 * Auto detect links with Python2 when both devtools are installed
 * Sensible warning is printed if no Python devtools are installed
 * 'make PYTHON=x' still automatically sets PYTHON_CONFIG=x-config
 * 'make PYTHON=x' fails if x-config doesn't exist
 * 'make PYTHON=python3' overrides Python2 devtools
 * 'make PYTHON=python2' overrides Python3 devtools
 * 'make PYTHON_CONFIG=x-config' works
 * 'make PYTHON=x PYTHON_CONFIG=x' works
 * 'make PYTHON=missing' reports an error
 * 'make PYTHON_CONFIG=missing' reports an error

Fixes: 79373082

 ("perf python: Autodetect python3 binary")
Signed-off-by: James Clark <james.clark@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20220309194313.3350126-2-james.clark@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>