core: introduce BR2_ENABLE_RUNTIME_DEBUG

core: introduce BR2_ENABLE_RUNTIME_DEBUG

Some packages have optional runtime assertions, extra traces, or other
elements that can help in debugging problems. However, such runtime elements
can negatively influence performance.

In a test program performing 100K gRPC calls from a client to a local server
and receiving the returned response, we see following execution time:

    - runtime debug enabled: 1065 seconds
    - runtime debug disabled:  48 seconds

This is more than a factor 20 (!) difference. Analysis shows that the
problem mostly stems from libabseil-cpp (a dependency of gRPC) which enables
mutex deadlock analysis when the preprocessor flag 'NDEBUG' is not set,
which adds a 'backtrace()' call on every lock/unlock.  Potentially worse,
when libunwind is enabled and linked with the test program, 'backtrace()' is
not provided by glibc but by libunwind itself.

For production systems, users expect good performance out-of-the-box. In the
example above, the difference is huge and unless explicitly tested and
analyzed, users may not realize that the performance could be much better.

Address this problem by introducing a new option BR2_ENABLE_RUNTIME_DEBUG,
which can be used by packages or package infrastructures to set the
necessary flags.

Note that BR2_ENABLE_RUNTIME_DEBUG is orthogonal to BR2_ENABLE_DEBUG: the
former changes runtime behavior, while the latter is only expected to add
debug symbols to the build. Today, the cmake build system does introduce a
runtime impact when BR2_ENABLE_DEBUG is set, but that will be rectified in a
subsequent commit.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>