Add __WEAK attribute to DSP Functions

[renesas-kyle-finch]

There are some cases where we need to use our own implementations of some arm_* functions.

If these functions had the __WEAK attribute, this would be possible without needing to customize CMSIS-DSP on a case by case basis.

[christophe0606]

@renesas-kyle-finch May I ask which functions need to be customized ?
What are the needed changes ?

Perhaps those changes may be integrated in the original function.

[renesas-kyle-finch]

@christophe0606 We have 32-bit multiply accumulator hardware in which we would like to use CMSIS DSP API functions that make use of our hardware registers within some of the DSP functions.

In our case, we have added __WEAK to the following functions:
arm_mult_q31
arm_scale_q31
arm_biquad_cascade_df1_q31
arm_conv_partial_q31
arm_conv_q31
arm_correlate_q31
arm_fir_decimate_q31
arm_fir_interpolate_q31
arm_fir_q31
arm_fir_sparse_q31
arm_lms_norm_q31
arm_lms_q31
arm_mat_mult_q31
arm_mat_scale_q31
arm_mat_vec_mult_q31

[christophe0606]

@renesas-kyle-finch Thanks for feedback. So, indeed in this case __WEAK is the only solution.
I'll plan it for a future release.