Document the use of `TX_LINUX_MULTI_CORE` in `ports/linux/gnu/readme_threadx.txt` or remove altogether

[jleveque]

Is your feature request related to a problem? Please describe.

Currently the TX_LINUX_MULTI_CORE #define is not documented, and without knowledge of the existence of this flag, trying to compile and run ThreadX on a multi-core Linux system can cause strange behaviors that are hard to track down due to threads running simultaneously on different cores.

Describe the solution you'd like

Update ports/linux/gnu/readme_threadx.txt with a section describing the TX_LINUX_MULTI_CORE #define and also the need to specify -D_GNU_SOURCE to have access to the cpu_set_t type and the CPU_COUNT, CPU_SET, and CPU_ZERO macros.

Describe alternatives you've considered

Alternatively, consider removing the TX_LINUX_MULTI_CORE #define altogether and unconditionally execute the code in _tx_initialize_low_level() which checks the number of cores and, if >1, will select a single core at random. Note that this solution would still require the developer to speficy -D_GNU_SOURCE to have access to the cpu_set_t type and the CPU_COUNT, CPU_SET, and CPU_ZERO macros, so this would still require documentation.

[fdesbiens]

Hi @jleveque.

Thank you for submitting this issue. I will discuss it with the project team.

Is there anyone in the @eclipse-threadx/iot-threadx-committers and @eclipse-threadx/iot-threadx-contributors groups willing to work on this one?

[hnguyenHWI]

Closing and moving this issue to Done, as PR #449 has been merged.

[jleveque]

Thank you very much for taking care of this, @hnguyenHWI!